Non-isolated DC-DC converter with power switch short circuit detection

Abstract

A non-isolated DC-DC converter detects a short circuit of its switching devices to avoid the occurrence of faults and unsafe operating conditions. The converter includes a transformer and a switching circuit including an upper switch and a middle switch of a primary side inverter and a lower switch of a secondary side rectifier. The short-circuit detection circuit has (1) a resistive voltage divider for establishing normal values of the primary-side test node and the secondary-side test node in a non-short-circuit situation with all switches in an off-state during a pre-operation, and (2) a sensing circuit for sensing the normal values of the primary-side test node and the secondary-side test node in a non-short-circuit situation during a pre-operation. And a detector operable during a pre-operation of the converter to detect an abnormal value of the primary side test node and the secondary side test node, the abnormal value indicating an abnormal short circuit of the upper switch, the middle switch, or the lower switch.

Description

TECHNICAL FIELD

[0001] The present invention relates to the field of switched DC-DC power converters. BACKGROUND

[0002] Power design and applications are continuously moving towards high efficiency, miniaturization, high frequency, and intelligence. Switched mode power supplies (SMPS) with small volume and light weight are widely used in computer, communication, power, instrument, home appliance, medical, and other fields. Because of the high efficient and reliable power output mode of SMPS, bridge type DC / DC converters have been widely used. These bridge type DC / DC converters include non-isolated full-bridge DC / DC converters, which are different from isolated DC / DC converters, and have advantages such as low cost, high efficiency, and high power density, thereby providing better solutions for applications in the field of high power. SUMMARY

[0003] A non-isolated DC-DC converter is disclosed that includes the ability to detect short circuits of its switching devices so that fault and potentially unsafe operating conditions can be avoided. The converter includes a transformer having a primary winding and a secondary winding, and a switching circuit including upper and middle switches of a primary side inverter and lower switches of a secondary side rectifier. The primary side inverter has a primary side test node between the upper and middle switches and is directly connected to the secondary side rectifier to provide a converter output current that is the sum of a primary current and a secondary current during an on interval of a switching cycle. The converter further includes a short circuit detection circuit having (1) a resistive voltage divider to establish a normal value of the primary side test node and a secondary side test node under non-short circuit conditions with all of the switches in an off state during pre-operation of the converter, and (2) a sensing circuit operable to detect an abnormal value of the primary side test node and the secondary side test node during the pre-operation of the converter that is indicative of an abnormal short circuit of the upper switch, the middle switch, or the lower switch. BRIEF DESCRIPTION OF DRAWINGS

[0004] The foregoing and other objects, features and advantages will be apparent from the following description of particular embodiments of the application, as illustrated in the accompanying drawings in which like reference characters refer to the same parts throughout the different views.

[0005] Figure 1 is a schematic diagram of a non-isolated DC-DC converter;

[0006] Figure 2 is a redrawn Figure 1 schematic diagram of the converter of

[0007] Figure 3 is a test circuit for testing for short circuits of switching devices such asFigure 2 A simplified flowchart of the technique for short-circuiting switching transistors in circuits such as [the circuit name is missing].

[0008] Figure 4 This is a schematic diagram of a converter that uses a first-stage three-stage voltage divider technology to test the short circuit of the switching transistor;

[0009] Figure 5 yes Figure 4 A schematic diagram of a variant of the converter that uses a blocking capacitor in the primary side of the transformer and an additional resistor to enable short-circuit testing;

[0010] Figure 6 This is a schematic diagram of a converter that uses a second- or third-stage voltage divider to test the short circuit of a switching transistor.

[0011] Figure 7 This is a schematic diagram of a converter that uses a two-stage voltage divider technique to test the short circuit of a switching transistor;

[0012] Figure 8 yes Figure 7 The flowchart shows the testing process for the converter. Detailed Implementation

[0013] SUMMARY; PROBLEMS SOLVED

[0014] Non-isolated full-bridge DC / DC converters can be classified into two types based on their connection method: traditional non-isolated control and optimized non-isolated control. Traditional non-isolated control involves directly connecting the input ground of a traditional isolated full-bridge DC / DC converter to the output ground. Under this condition, the converter's efficiency, power density, and cost are similar to those of the isolated version.

[0015] The improved non-isolated control features a more optimized circuit topology. The improved non-isolated full-bridge converter offers advantages such as simple circuit structure and low cost, effectively reducing the transformer turns ratio while lowering the current stress on the rectifier switches, and enabling higher power density. However, the improved non-isolated full-bridge converter also increases the risk of potential faulty electrical connections between the input and output (a serious fault condition that should be avoided).

[0016] Based on the above advantages, the improved non-isolated full-bridge DC / DC converter is applied in the field of switch-mode power supplies. However, since there is no electrical isolation between the input and output, there is a risk of direct connection between the input and output. To protect the converter from a fault condition where the input voltage shoots through the output voltage, a simple circuit and method are employed to detect short circuits in the switching devices before the converter starts up.

[0017] EMBODIMENTS

[0018] Figure 1 A non-isolated full-bridge converter is shown, comprising a transformer T1, primary-side switches Q1 to Q4 in a full-bridge configuration, secondary-side switches Q5 to Q6, and other components including capacitors Cin and Cout, inductor Lout, and a load representation as a load resistor Rld. Transformer T1 has a primary winding TP and two secondary windings T-S1 and T-S2. Both the primary and secondary circuits are connected to a single common ground GND. This common ground connection between the primary and secondary circuits establishes the "non-isolated" aspect of this arrangement. It will be understood that the gates of transistors Q1 to Q6 are each connected to a corresponding switch control signal from a separate control circuit, which, although not explicitly shown, will be understood by those skilled in the art to have a structure and function for implementing the operations fully described below.

[0019] More specifically:

[0020] 1. The primary bridge consists of Q1, Q2, Q3, and Q4. The drains of Q1 and Q2 are connected together and connected to the Vin source. The source of Q1 is connected to the drain of Q3 and to one end of the primary winding TP of transformer T1 (voltage node V). A The source of Q2 is connected to the drain of Q4, and also to the other end of the primary winding TP of transformer T1; and the sources of Q3 and Q4 are connected together (voltage node V). B );

[0021] 2. Transformer T1 has a primary winding TP and secondary windings T-S1 and T-S2, wherein windings TP, T-S1 and T-S2 are fully coupled to each other;

[0022] 3. The output rectifier circuit includes Q5 and Q6, wherein Q5 is connected in series with T-S1 and Q6 is connected in series with T-S2;

[0023] 4. The source nodes of Q3 and Q4 are bonded together and connected (via connection 10) to the output inductor Lout and the secondary center tap between T-S1 and T-S2;

[0024] 5. The output filter includes an inductor Lout and a capacitor Cout;

[0025] 6. The primary winding TP has Np turns, and the secondary windings T-S1 and T-S2 have the same number of turns and are labeled Ns.

[0026] The overall operation is a series of ongoing switching cycles with regular intervals of duration T, corresponding to a switching frequency, which can be, for example, in the range of 10 to 100 kHz. Each cycle is divided into multiple periods or intervals, including two on periods (duration D) separated by two off periods (duration TD), where the on period refers to primary-side current conduction and the off period refers to primary-side current non-conduction. A characteristic of the non-isolated arrangement is that during each on period, the input current ITP flows to the output along with the secondary-side current, such that the total output current equals the sum of these currents. The secondary-side circuitry does not need to carry all the output current. Advantages include: for a given output voltage, it is possible to use a reduced transformer turns ratio, thereby reducing circuit cost and improving efficiency; and for a given load, due to the reduced secondary-side current, it is possible to use smaller secondary-side components, thereby also reducing cost and improving efficiency.

[0027] However, as mentioned above, in a non-isolated arrangement, there is a risk of faulty electrical connections between the input and output, especially in the event of a short circuit in the switching transistors (Q1 to Q6). To protect the converter from this fault and potentially unsafe operating condition where the input voltage shoots through the output voltage, a simple circuit and method for detecting short circuits in the switching transistors are proposed.

[0028] Figure 2 It's a redraw. Figure 1 The non-isolated converter is used to emphasize certain aspects of its construction. As shown in the figure, the switching transistors are divided into three types:

[0029] Q1 to Q2: Upper group, between Vin and V A between

[0030] Q3 to Q4: Middle group, between V A and B between

[0031] Q5 to Q6: Next group, between V B Between GND

[0032] There are three different short-circuit fault conditions to be detected: a short circuit in one of the upper switches Q1 to Q2, a short circuit in one of the middle switches Q3 to Q4, and a short circuit in one of the lower switches Q5 to Q6. For the upper switches Q1 to Q2, when any of these switches is short-circuited and the converter is not operating (e.g., in standby mode), node V... A It is actually directly connected to Vin (via the primary winding TP in the case of Q2 short circuit), so node V A The voltage will be close to the input voltage Vin. In the event of a short circuit in one of the switches Q3 to Q4, node V...A The output inductor Lout (via the primary winding TP when Q4 is short-circuited) is connected to Vout, therefore node V A The voltage will be close to the output voltage Vout. When one of the switches Q5 to Q6 is short-circuited, the voltage at node V... B This will short-circuit to ground through the corresponding secondary winding T-S1 or T-S2, thereby causing V B The voltage is close to 0. Therefore, based on checking V A or V B A testing technique is proposed based on any of these fault short-circuit voltages (which are distinct from the normal voltage values ​​in the absence of a short circuit).

[0033] Figure 3 This is a simplified flowchart of a testing technique in one embodiment. At point 10, the technique includes incorporating a resistor divider into the converter to establish a primary-side test node (e.g., node V). A ) and secondary side test nodes (e.g., node V) B The normal values ​​are the voltage values ​​under non-short-circuit conditions with all switches off during the pre-operation of the converter. At 12, the sensing circuit is capable of operating during the pre-operation of the converter to detect abnormal values ​​at the primary-side test node and the secondary-side test node, indicating abnormal short circuits in one or more of the upper, middle, and lower switches. In one method, such abnormal values ​​are detected by comparing the actual test node voltage with the corresponding short-circuit voltage under expected short-circuit conditions (e.g., VA = Vin or VB = 0).

[0034] Several alternative methods are described below. These methods all have a self-referencing aspect, i.e., comparing the voltage to the normal operating value of a given converter. For different non-isolated full-bridge DC / DC power supplies, the input voltage, output voltage, and transformer turns ratio typically differ. To effectively distinguish the state of the upper, middle, or lower switch under short-circuit and normal conditions, additional circuitry is added to adjust the node voltages before starting the converter to differentiate between normal and short-circuit conditions.

[0035] 1. Three-stage voltage division method

[0036] Figure 4 An example of the first test method, known as the "three-stage voltage divider method," is shown. Resistors R1, R2, and R3 are added to the converter circuit in the form of a voltage divider as shown in the figure. The test is based on detecting V... A The value is used to distinguish whether the upper and middle switches are in a normal or short-circuit state, and based on the detection V BThe value is used to distinguish whether the switch is in a normal or short-circuit state. The test is performed by the sensing circuit (CKTRY) 20 when the converter is in standby mode (before startup). In one embodiment, the sensing circuit 20 is integrated into a control circuit (not shown) that manages the overall operation of the converter.

[0037] When switching transistor Q1 or Q2 is short-circuited, V A The voltage equals Vin. When switching transistors Q3 or Q4 are short-circuited, V... A The voltage is equal to Vout. Resistors R1, R2, and R3 are used to establish the voltage Vout. A and V B The normal values ​​(i.e., the voltage values ​​under conditions without short circuits) are typically much smaller than the static drain-source turn-off resistances Rdsoff of switches Q1 through Q6. Furthermore, to ensure that no higher voltage is directly divided through Vin to Vout in the non-operating state, the resistance values ​​of R1 and R2 should be much larger than R3. Additionally, for R3 to provide a measurable effect, its resistance must be much smaller than the grounding resistance Rld. Specific ranges for these resistance values ​​are given below.

[0038] When the converter is in standby mode, in the absence of a short circuit, node V A The normal voltage (VAnom) is shown below.

[0039] The following conditions must be met to distinguish between a short circuit in the upper or middle switch and a normal non-short circuit.

[0040] Right now, , ,

[0041] When the converter is in standby mode and the lower switch is short-circuited, V B The voltage is close to 0. If there is a large pre-bias voltage at Vout (i.e., Vout is much higher than 0 volts), this means that the lower switches Q5 through Q6 will not be short-circuited. If there is no large bias voltage at Vout, then the voltage will be similar to that under normal (non-short-circuit) conditions. B The corresponding voltages are shown below:

[0042] The following conditions must be met to distinguish the short circuit of the lower switch from its normal state.

[0043] When the converter is in standby mode, Rld is the impedance between Vout and ground. When Rld is in the 10 kΩ range and Rdsoff is in the 10 kΩ range or higher, the impedances of R1 and R2 can be set in the approximately 100 kΩ range, and the impedance of R3 should be in the 1 kΩ range. The ratio of R1 to R2 should be selected such that Vout... A Distinguishing between Vin and Vout allows for the detection of short circuits in the upper or middle switch. A good ratio of R1 to R2 is 1:1. As long as V... B If the voltage after voltage division can meet the sampling requirements and be distinguished from zero voltage, then the normal state and short circuit can be distinguished.

[0044] Assume V A and V B If the above-mentioned normal values ​​are true, then the following conditions during startup indicate a short circuit:

[0045] 1.V A =Vin: Q1 or Q2 short circuit

[0046] 2.V A =Vout: Q3 or Q4 short-circuited

[0047] 3.V B =0: Q5 or Q6 short circuit

[0048] Although the above recommendation suggests performing three separate tests for the corresponding fault indication value, in another embodiment, each voltage V A V B It can be compared with its corresponding normal / nominal values ​​VAnom and VBnom, and if both voltages have their normal / nominal pre-operation values, no short circuit is detected and normal operation of the converter can be enabled.

[0049] Figure 5 It shows Figure 3 A variant of the circuit employs capacitor C1 to prevent imbalance in transformer T1 during normal switching operation of the converter. The presence of this capacitor eliminates the imbalance at node V. A The DC path between switches Q3 and Q4 complicates the detection of short circuits in these devices. To achieve this, V... A To detect the anomaly in Q3 and Q4, resistor R4 was added, as follows: Figure 5 As shown. The resistance of R4 should be much smaller than the resistances of R1 and R2 (for example, when R1 and R2 are 10 kΩ, the resistance of R4 is about 1 kΩ). By adding R4, the above-mentioned... Figure 4 Testing techniques for circuit descriptions.

[0050] Figure 6A three-stage voltage divider method is shown, which, except that it does not require resistor R3, is similar to the method described above. Figure 4 The three-stage voltage divider method is similar. This is possible because a grounding impedance Rld exists in general circuit designs. The technique is modified to rely on the normal voltage established by Rld combined with R1 and R2.

[0051] exist Figure 6 In the circuit, when the switching transistor Q1 or Q2 is short-circuited, V A The voltage equals Vin. When switching transistors Q3 or Q4 are short-circuited, V... A The voltage is equal to Vout. Resistors R1 and R2 are used to establish the voltage Vout. A and V B The normal values ​​(i.e., the voltage values ​​under conditions without short circuits) are typically much smaller than the static drain-source turn-off resistors Rdsoff of switches Q1 through Q6. Furthermore, to ensure that no higher voltage is directly divided through Vin to Vout in the non-operating state, the resistance values ​​of R1 and R2 are much larger than Rld. Specific ranges for these resistance values ​​are given below.

[0052] In the absence of a short circuit and with the converter in standby mode, V A The voltage is shown below.

[0053] The following conditions must be met to distinguish the short circuit of the upper or middle switch from its normal state.

[0054] Right now, , ,

[0055] When the converter is in standby mode and the lower switch Q5 or Q6 is short-circuited, the Vout voltage is close to 0. If there is a large pre-bias voltage at Vout, neither Q5 nor Q6 will be short-circuited. If there is no high bias voltage at Vout, the normal output voltage Voutnom during normal operation is as follows:

[0056] The following conditions must be met to distinguish the short circuit of the lower switch from its normal state:

[0057] When Rld is in the 10 kΩ range and Rdsoff is in the 10 Mohm range or higher, R1 and R2 are preferably in the approximately 100 kΩ range. The ratio of R1 to R2 should be selected such that V AThis is to distinguish Vin and Vout, allowing for the detection of short circuits in the upper or middle switch. A good ratio of R1 to R2 is 1:1. The voltage after Vout is divided can be distinguished from a short circuit as long as it meets the sampling requirements and is distinguishable from 0 voltage.

[0058] 2. Two-stage voltage division method:

[0059] Figure 7 An example of a second test method, known as the "two-stage voltage divider method," is shown. To prevent a small voltage across Vout from Vin directly to Vout before startup, resistor R2 is directly connected to GND, unlike the three-stage voltage divider method. R1 and R2 are used to sense Vout. A To distinguish whether the upper and middle switches are in a normal or short circuit state, and to use Vout to distinguish whether the lower switch is in a normal or short circuit state.

[0060] exist Figure 7 During the pre-operation period of the circuit, when switching transistor Q1 or Q2 is short-circuited, V A The voltage equals Vin. When switching transistors Q3 or Q4 are short-circuited, V... A The voltage is equal to Vout. Resistors R1 and R2 are used to establish the voltage Vout. A and V B The normal value (i.e., the voltage value under conditions where there is no short circuit). Typically, the resistance values ​​of R1 and R2 are much smaller than the static drain-source turn-off resistance Rdsoff of switches Q1 through Q6.

[0061] When the switch is functioning normally and the converter is in standby mode, V A The voltage is shown below:

[0062] The following conditions must be met to distinguish the short circuit of the upper or middle switch from its normal state:

[0063] Right now, , ,

[0064] When switch Q5 or Q6 is short-circuited and the converter is in standby mode, the Vout voltage is close to 0. In static standby operation, if there is a large pre-bias voltage at Vout, neither switch Q5 nor Q6 will short-circuit. However, if there is no high bias voltage at Vout, the corresponding voltage at Vout when switches Q5 and Q6 are normal is 0 (due to the absence of R3), which is the same as in the short-circuit case. Therefore, to distinguish between the normal and short-circuit conditions, a pulse operation is used to drive Vout to a measurable non-zero test voltage through the full-bridge section of the circuit. Vout is then sensed to determine whether it has reached such a non-zero test voltage (indicating no short circuit) or remains at 0 (indicating a short circuit).

[0065] Figure 8 The above technique is illustrated and can be executed by a converter controller including sensing circuit 20. At 30, it is determined whether Vout is equal to 0; if not, the lower switches are normal (not short-circuited), and thus normal converter startup can be performed. If Vout is zero, at 32, several on-pulses are supplied to switches Q1 to Q4 to drive Vout to a small test value, as described above. Subsequently, the PWM pulses are disabled at 34, and a small delay is observed at 36. Then, at 38, Vout is compared to zero again. If Vout is non-zero, the lower switches Q5 and Q6 are normal (not short-circuited), and thus normal converter startup can be performed. If Vout is equal to zero, it indicates that one (or both) of the lower switches are short-circuited, and this indication can be used to signal a fault condition and prevent normal converter startup.

[0066] Refer again Figure 6 and Figure 7 The technology will be understood to also be used when the converter includes a blocking capacitor C1, simply by means of... Figure 5 The parallel resistor R4 shown is sufficient.

[0067] While various embodiments of the invention have been specifically shown and described, those skilled in the art will understand that various changes in form and detail may be made therein without departing from the scope of the invention as defined by the appended claims.

Claims

1. A non-isolated DC-DC converter comprising: a transformer having a primary winding and a secondary winding; a switching circuit including upper switches and middle switches of a primary side inverter having a primary side test node between the upper switches and the middle switches and a lower switch of a secondary side rectifier connected directly to the primary side inverter to provide a converter output current as a sum of a primary current and a secondary current during an on interval of a switching cycle; and a short circuit detection circuit including (1) a resistive divider to establish normal values of the primary side test node and a secondary side test node under non-short circuit conditions with all of the switches in an off state during pre-operation of the converter and (2) a sensing circuit operable to detect abnormal values of the primary side test node and the secondary side test node during pre-operation of the converter, the abnormal values indicating an abnormal short circuit of the upper switches, the middle switches, or the lower switch. The resistive divider is a three-stage divider having a set of series resistors, each series resistor in parallel with a respective switch of the upper switches, the middle switches, and the lower switch, and wherein the primary side test node is a first node between a first resistor and a second resistor of the resistors having a first normal voltage between an input voltage Vin and an output voltage Vout of the converter and the secondary side test node is a second node between the second resistor and a third resistor of the resistors having a second normal voltage between the input voltage and the output voltage of the converter, and wherein the abnormal values of the primary side test node and the secondary side test node are: (1) the primary side test node being Vin indicating one of the upper switches shorted; (2) the primary side test node being Vout indicating one of the middle switches shorted; (3) the secondary side test node being 0 indicating one of the lower switches shorted.

2. The non-isolated DC-DC converter of claim 1, wherein, The short circuit detection circuit includes an additional resistor in parallel with the blocking capacitor to provide a current path such that the value of the primary side test node is the Vin value in the event of the one upper switch shorted.

3. The non-isolated DC-DC converter of claim 2, comprising a primary side blocking capacitor for preventing an imbalance of the transformer in operation, the blocking capacitor being connected in series between the primary side test node and one of the upper switches, and wherein, The resistive divider is a three-stage divider having a set of series resistors, each series resistor in parallel with a respective switch of the upper switches and the middle switches, and wherein the primary side test node is a first node between a first resistor and a second resistor of the resistors having a first normal voltage between an input voltage Vin and an output voltage Vout of the converter and the secondary side test node is an output node of the converter, and wherein the abnormal values of the primary side test node and the secondary side test node are: (1) the primary side test node being Vin indicating one of the upper switches shorted; (2) the primary side test node being Vout indicating one of the middle switches shorted; (3) the output node of the converter being 0 indicating one of the lower switches shorted.

4. The non-isolated DC-DC converter of claim 1, wherein, ​ 5. The non-isolated DC-DC converter of claim 1, wherein, The resistor divider is a two-stage resistor divider with a set of series resistors, where a first one of the resistors is connected in parallel with the upper switches, and a second one of the resistors is connected between the first resistor and ground, and where the primary-side test node is a first node between the first and second resistors, having a first normal voltage between an input voltage Vin and an output voltage Vout of the converter, and where the secondary-side test node is an output node of the converter, and where abnormal values of the primary-side test node and the secondary-side test node are: (1) the primary-side test node being Vin, indicating a short in one of the upper switches; (2) the primary-side test node being Vout, indicating a short in one of the middle switches; (3) the output node of the converter being 0, indicating a short in one of the lower switches, the 0 value being detected after a pulse operation by the sensing circuit effectively drives Vout to a measurable non-zero value in the absence of a short in the lower switches.

6. A method of testing a non-isolated DC-DC converter for shorts in any of upper switches, middle switches, and lower switches of the converter, the method comprising: incorporating a resistor divider to establish normal values of a primary-side test node and a secondary-side test node in the absence of a short during pre-operation of the converter with all of the switches in an off state; and operating a sensing circuit during pre-operation of the converter to detect abnormal values of the primary-side test node and the secondary-side test node, the abnormal values indicating an abnormal short in the upper switches, the middle switches, or the lower switches.

7. The method of claim 6, wherein, The resistor divider is a three-stage resistor divider with a set of series resistors, each series resistor being connected in parallel with a respective one of the upper switches, the middle switches, and the lower switches, and where the primary-side test node is a first node between a first one of the resistors and a second one of the resistors, having a first normal voltage between an input voltage Vin and an output voltage Vout of the converter, and the secondary-side test node is a second node between the second one of the resistors and a third one of the resistors, having a second normal voltage between the input voltage and the output voltage of the converter, and where detecting abnormal values of the primary-side test node and the secondary-side test node includes: (1) detecting the primary-side test node being Vin, indicating a short in one of the upper switches; (2) detecting the primary-side test node being Vout, indicating a short in one of the middle switches; (3) detecting the secondary-side test node being 0, indicating a short in one of the lower switches.

8. The method of claim 7, wherein, The converter includes a primary-side blocking capacitor to prevent transformer imbalance in operation, the blocking capacitor being in series between the primary-side test node and one of the upper switches, and the converter further includes an additional resistor in parallel with the blocking capacitor to provide a current path such that the primary-side test node has a value of Vin in the event of a short in the one upper switch.

9. The method of claim 6, wherein, The resistive voltage divider is a two-stage voltage divider with a set of series resistors, a first of the resistors being in parallel with the upper switches, and a second of the resistors being connected between the first resistor and ground, and wherein the primary-side test node is a first node between the first and second resistors, having a first normal voltage between an input voltage Vin and an output voltage Vout of the converter, and wherein detecting abnormal values of the primary-side test node and the secondary-side test node includes: (1) detecting that the primary-side test node is Vin, indicating a short in one of the upper switches; (2) detecting that the primary-side test node is Vout, indicating a short in one of the middle switches; (3) detecting that the output node of the converter is 0, indicating a short in one of the lower switches, the 0 value being detected after a pulse operation by the sensing circuit effectively drives Vout to a measurable non-zero value in the absence of a short in the lower switches.

10. The method of claim 6, wherein, The resistive voltage divider is a two-stage voltage divider with a set of series resistors, a first of the resistors being in parallel with the upper switches, and a second of the resistors being connected between the first resistor and ground, and wherein the primary-side test node is a first node between the first and second resistors, having a first normal voltage between an input voltage Vin and an output voltage Vout of the converter, and wherein detecting abnormal values of the primary-side test node and the secondary-side test node includes: (1) detecting that the primary-side test node is Vin, indicating a short in one of the upper switches; (2) detecting that the primary-side test node is Vout, indicating a short in one of the middle switches; (3) detecting that the output node of the converter is 0, indicating a short in one of the lower switches, the 0 value being detected after a pulse operation by the sensing circuit effectively drives Vout to a measurable non-zero value in the absence of a short in the lower switches.

11. The method of claim 10, wherein, The pulse operation is performed after first detecting that the output node is 0, and the pulse operation includes: applying a plurality of on pulses to the upper switches and the middle switches in a manner effective to drive Vout to the measurable non-zero value in the absence of a short in the lower switches; next disabling all pulse width modulation of the switches, then delaying for a predetermined time; next detecting whether Vout remains at 0, Vout remaining at 0 indicating a short in one of the lower switches.

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