How to operate a DC / DC converter

Abstract

A method for operating a DC / DC converter (10). The DC / DC converter (10) comprises a transformer (3) having a primary side (1), a secondary side (2), and a center tap (39). The upper transformer tap (33) and the lower transformer tap (34) are connected via a full bridge circuit to the upper terminal point of a snubber capacitance (45) by upper switches (35, 37) and to the lower terminal point of the snubber capacitance (45) by lower switches (36, 38). The lower secondary DC terminal (48) is connected to the lower terminal point of the snubber capacitance (45), and the upper secondary DC terminal (47) is connected to the center tap (39) via a filter inductance (43). In this method, under certain operating conditions, the upper switches (35, 37) alternately apply positive voltage pulses of the secondary voltage between the center tap (39) and the upper terminal point of the snubber capacitance (45). Between voltage pulses, the upper switches (35, 37) are closed and the lower switches (36, 38) are open.

Description

[Technical Field]

[0001] The present invention relates to the field of electronic power converters. The present invention relates to a method for operating a DC / DC converter, a control unit for a DC / DC converter, and a DC / DC converter, as described in the preamble of each of the independent claims.

[0002] The power electronics market has a high demand for low-power DC / DC converters, such as those used to convert high voltage (HV) to low voltage (LV), for applications like supplying power to low-voltage batteries, for example, around 3kW. The voltage of such low-voltage batteries is typically in the range of 10.5V to 15.5V. The converter must operate across a wide input voltage range, ideally without power reduction. Furthermore, it is desirable to have as many features as possible and a very low price point. Electric vehicle manufacturers are tending to increase the voltage of high-voltage batteries, and customer requirements may vary. Starting with today's 400V battery voltages, 600V to 800V battery voltages will likely become the new standard. Corresponding DC / DC converters should cover the widest possible voltage range to ensure broad applicability.

[0003] Figure 1 shows the primary input voltage V p From the secondary output voltage V sThe structure of such a DC / DC converter is shown in FIG. Therein, two full bridges 19, 40 coupled by a transformer 3 are shown. The transmitted power is controlled by the intermittent application of a voltage to the primary side of the transformer. The output voltage is closed-loop controlled by the ratio (duty cycle) of the on-time and off-time of this voltage on the primary side. The two lower switches S6 and S8 on the secondary side rectify the voltage applied to the transformer and serve to short-circuit the secondary transformer winding in the free-wheeling state. In FIG. 2, FIG. 2(a) shows typical waveforms at a duty cycle of 25%. The two upper switches S5 and S7 on the secondary side are only used as snubs (dampers) for the high voltage peaks on the secondary side caused by the leakage inductance of the transformer. In the steady state, the voltage at the center tap 39 of the transformer 3 is equal to the secondary side output voltage, that is, <v ct >=V s Thus, the DC input / output transfer function M(d) is given by

Equation

[0004]

[0005] Therefore, an object of the present invention is to provide a method for operating a DC / DC converter of the type described at the beginning, a control unit for a DC / DC converter, and a DC / DC converter that enable a wider range of application fields. Thereby, in particular, when a given voltage is present on the output side, it should be possible to operate with a wider range of input voltages.

[0006] This objective is achieved by a method for operating a DC / DC converter, a control unit for a DC / DC converter, and a DC / DC converter, each having the features of its respective independent claim.

[0006] This method is for operating a DC / DC converter. This DC / DC converter comprises a transformer with primary, secondary, and center taps. The upper and lower transformer taps are connected via a full-bridge circuit to the upper terminal point of a snubber capacitance by an upper switch and to the lower terminal point of a snubber capacitance by a lower switch. The lower secondary DC terminal is connected to the lower terminal point of a snubber capacitance, and the upper secondary DC terminal is connected to the center tap via a filter inductance.

[0007] In this first operating state, the lower switch alternately applies positive voltage pulses of the secondary voltage between the center tap and the lower secondary DC terminal with a duty cycle d between 0 and 0.5, during which the lower switch is closed, and the upper switch diverts the current that would lead to the voltage peak when switching the lower switch off to a snubber capacitance.

[0008] In the second operating state, the upper switch alternately applies positive voltage pulses of the secondary voltage between the center tap and the upper terminal point of the snubber capacitance, and between voltage pulses, the upper switch is closed and the lower switch is open.

[0009] In this way, in the second operating state, when the transformer is current-free on the primary side, the first upper switch and the second upper switch are closed, thereby clamping the voltage at the center tap to the voltage at the first terminal point of the snubber capacitance.

[0010] In the second operating state, when the upper switch is closed during the voltage pulse, the voltage at the upper secondary DC terminal is formed from the voltage across the snubber capacitor. As a result, the voltage range that the output voltage can cover as a whole is doubled. This doubling is with respect to the maximum voltage achievable in the first operating state where the lower switch remains closed and the duty cycle is limited to 0.5.

[0011] In an embodiment, in the second operating state, following the application of a voltage pulse by closing one of the upper switches, in each case, to limit the discharge of the snubber capacitor, this upper switch is opened again.

[0012] As a result, the voltage across both ends of the snubber capacitor begins to oscillate.

[0013] A method for operating a DC / DC converter will be described in more detail below. The DC / DC converter includes a primary side, a secondary side, and a transformer, · The transformer includes an upper transformer tap, a lower transformer tap, and a center tap on the secondary side, · The secondary side includes a lower secondary DC terminal and an upper secondary DC terminal, · A filter inductance is connected between the upper secondary DC terminal and the center tap, · A first upper switch is connected between the upper transformer tap and a first terminal point of a snubber capacitor whose second terminal point is connected to the lower secondary DC terminal, · A second upper switch is connected between the lower transformer tap and the first terminal point of the snubber capacitor, · A first lower switch is connected between the upper transformer tap and the lower secondary DC terminal, · A second lower switch is connected between the lower transformer tap and the lower secondary DC terminal, · Here, each of the switches includes a freewheel diode.

[0014] Here, in the extended operating state, the DC / DC converter repeatedly executes the following states in this order. · First state: The transformer has no current on the primary side, the first upper switch and the second upper switch are closed, and the first lower switch and the second lower switch are open. · Second state: The transformer generates a positive voltage on the upper transformer tap with respect to the lower transformer tap, the second upper switch is open, and the second lower switch is closed. · Third state: The first upper switch is opened. · Fourth state: The transformer is switched to no current on the primary side, the first upper switch and the second upper switch are closed, and the second lower switch is open. · Fifth state: The transformer generates a negative voltage on the upper transformer tap with respect to the lower transformer tap, the first upper switch is open, and the first lower switch is closed. · Sixth state: The second upper switch is opened. The third state and the sixth state are optional, that is, they are skipped in the embodiment.

[0015] In an embodiment, the time when the first state and / or the fourth state is active is

Number

[0016] In an embodiment, the time when the second state and / or the fifth state is active is

Number

[0017] In an embodiment, when a duty cycle exceeding 0.5 is given, the transmission power is reduced. In an embodiment, the transmission power is reduced by reducing the output current. As a result, this method can be implemented in existing circuit hardware, and the voltage ratio that can be transmitted can be increased without changing the hardware.

[0018] The control unit for a DC / DC converter according to the present invention is programmed to execute the method according to any one of the preceding claims. The control unit further includes a sensor for detecting current and / or voltage in the DC / DC converter, and a control stage for controlling the switches of the DC / DC converter. The DC / DC converter according to the present invention includes such a control unit.

[0019] A computer program for operating the control unit for a DC / DC converter according to the present invention can be loaded into the internal memory of the digital data processing unit of the control unit, and when implemented in the control unit, includes computer program code means for causing the control unit to execute the method according to the present invention. In a preferred embodiment of the present invention, the computer program product includes a data carrier, that is, a computer-readable medium on which the computer program code means are stored.

[0020] Further preferred embodiments are derived from the dependent claims. Here, contextually, the features of the method claims can be combined with the apparatus claims, and vice versa.

Brief Description of the Drawings

[0021] The subject matter of the present invention will be described in more detail below with reference to examples of preferred embodiments shown in the accompanying drawings. Each figure schematically illustrates the following: [Figure 1] Figure 1 is a schematic diagram showing the structure of a DC / DC converter. [Figure 2] Figure 2(a) schematically shows the voltage progression of the primary and secondary sides in the first operating state of the DC / DC converter, Figure 2(b) schematically shows the voltage progression of the primary and secondary sides in the second operating state of the DC / DC converter, and Figure 2(c) schematically shows the relationship between the duty cycle and the output voltage. [Figure 3] Figure 3 is a schematic diagram showing the signal progression in the second operating state. [Figure 4] Figure 4 schematically shows the reduction in transmission power in the second operating state.

[0022] Generally, in diagrams, parts that function identically or equivalently are given the same reference numeral. [Modes for carrying out the invention]

[0023] Figure 1 shows the structure of a DC / DC converter. The DC / DC converter 10 comprises a primary side 1, a secondary side 2, and a transformer 3. The primary side 1 has an upper primary side DC terminal 11 and a lower primary side DC terminal 12, and a primary side bridge circuit 19, powered through these terminals, applies voltage pulses to the primary side of the transformer 3.

[0024] The transformer 3 comprises an upper secondary winding 31 and a lower secondary winding 32, and has an upper transformer tap 33, a lower transformer tap 34, and a center tap 39 on the secondary side.

[0025] The secondary side 2 includes a lower secondary DC terminal 48 and an upper secondary DC terminal 47, and a filter inductance 43 is connected between the upper secondary DC terminal 47 and the center tap 39. The secondary bridge circuit 40 includes the following: A first upper switch 35 is connected between the upper transformer tap 33 and the first terminal point of the snubber capacity 45, the second terminal point of which is connected to the lower secondary DC terminal 48. • A second upper switch 37 is connected between the lower transformer tap 34 and the first terminal point of the snubber capacity 45. • A first lower switch 36 is connected between the upper transformer tap 33 and the lower secondary DC terminal 48. A second lower switch 38 is connected between the lower transformer tap 34 and the lower secondary DC terminal 48. Here, each of switches 35, 36, 37, and 38 is equipped with a freewheeling diode.

[0026] The lower secondary DC terminal 48 and the upper secondary DC terminal 47 are interconnected by a filter capacitance 44. That is, the filter capacitance 44 is connected between the lower secondary DC terminal 48 and the upper secondary DC terminal 47.

[0027] Figure 2(a) shows the voltage progression of the primary and secondary sides in the first operating state of the DC / DC converter. This corresponds to conventional control of the DC / DC converter. In the steady state, the output choke voltage averages to zero over the switching period, so the output voltage V of the DC / DC converter s This is the average center tap voltage. <vct>This becomes equal to. With regard to known hard-switch closed-loop control methods, the average voltage is closed-loop controlled by setting the duty cycle d of the primary voltage, as shown in Figure 2(a). In the prior art, both low-side MOSFETs (S6, S8) on the secondary side 2, i.e., the first lower switch 36 and the second lower switch 38, are switched on in a free-wheeling state.

[0028] Figure 2(b) shows the primary and secondary voltage paths in the second operating state of the DC / DC converter. Here, both high-side MOSFETs (S5, S7) on the secondary side 2, i.e., the first upper switch 35 and the second upper switch 37, are switched on (this is in contrast to the conventional technique where two upper switches are switched on, similar to the primary side). Depending on which MOSFET (low-side or high-side) is used during freewheeling, different voltage waveforms are produced at the center tap [Figures 2(a) and 2(b), respectively]. Therefore, using the second operating state ("high-side" freewheeling) allows for an increase in the output voltage range, in which case the snubber capacitance 45 is also used as an energy buffer.

[0029] Figure 2(c) shows the duty cycle and the output voltage or average center tap voltage. <vct>This shows the relationship. 0 and V p For output voltages between / n, the DC / DC converter operates in the first operating state. Primary duty cycle d pri This is equal to the duty cycle d of the DC / DC converter and takes a value between 0 and 0.5. ·V p / n and 2V p For output voltages between / n, the DC / DC converter operates in the second operating state. Primary duty cycle d pri This is equal to 0.5 - |0.5 - d|, where d takes a value between 0.5 and 1. The duty cycle d of the DC / DC converter sets the ratio of the output voltage to the maximum output voltage.

[0030] Therefore, the duty cycle d limit of the DC / DC converter, as shown on the left y-axis in Figure 2c), is removed. If the duty cycle value exceeds 0.5, the upper switches (first upper switch 35 and second upper switch 37) are used during freewheeling to short-circuit the secondary winding of the transformer. This allows an average center tap voltage of up to 2Vp / n to be applied. On the primary side, the duty cycle is within the initial range, i.e., 0 <d pri It remains <0.5. In contrast, the duty cycle d range is now 0 <d<1となり、d pri The relationship between and d is given by the following equation. d pri =0.5-|0.5-d|

[0031] To minimize the effective current of power transistors S5 and S7, the charging time of the snubber capacity 45 is set to the leakage inductance (L) of transformer 3. lp , L ls1 , L ls1 ) and the snubber capacitance 45 are preferably limited. While the primary side is active, each secondary side switch S5 or S7 is switched on for half the resonant time. This time is calculated as follows:

number

[0032] Switches S5 and S7 clamp the center tap to a voltage with a snubber capacitance of 45, thereby V p Output voltage V is higher than / n s To enable this, it remains active even while freewheeling.

[0033] In the freewheeling state, the snubber capacitance 45 is discharged by the current of the filter inductance 43. Therefore, the switch-on times for both switches S5 and S7 are subdivided into two intervals. Part of the time (T on1-S5S7 This can be calculated based on the required duty cycle, i.e., the time the switch needs to clamp the center tap to the snubber capacitance voltage.

number

[0034] As already explained, further parts of time (T on2-S5S7 This is based on the period of the oscillating circuit during the snubber time interval in which the snubber capacity is charged. T on2-S5S7 =T half_periоd

[0035] Figure 3 shows the signal progression in the second operating state, i.e., the PWM signal and transformer waveforms. The period is assumed to begin when S2 and S3 are switched off, as shown on the left side of Figure 3. Switch S5 is switched off at time T on1-S5S7 +T on2-S5S7 During the time S7 is switched on, on1-S5S7 It is switched on only during this time. After that, S5 is switched off to prevent the current in the snubber capacity 45 from oscillating and discharging the snubber capacity 45. At the start of the second half cycle, S5 is switched off to short-circuit the transformer during the freewheel interval T on1-S5S7 It needs to be switched on again only during this time. S7 also needs to be switched on at this point, time T on1-S5S7 +T on2-S5S7 It needs to be switched on during this time. S7 remains switched off for the remainder of the half-cycle. Then the cycle ends and begins anew with the switching on of S5 and S7. In Figure 3, the value of d is approximately 0.6, and d -pri The value is approximately 0.4.

[0036] In this embodiment, the snubber capacitance 45 is sufficiently large that it is not necessary to switch off either S5 or S7, which would be required to prevent the current in the snubber capacitance 45 from oscillating and discharging the snubber capacitance 45. This is particularly possible in situations where the DC / DC converter is designed to operate in the second operating state from the outset.

[0037] For DC / DC converters that are not primarily designed to operate in the second operating state, switches S5 and S7 are, in principle, not designed to have high current load capacities. Due to this constraint of the switches used, such converters must implement output power derating depending on the duty cycle d. This means that the transmitted power is reduced beyond a certain duty cycle. This allows for a wider output voltage range that is possible at full power up to this duty cycle, and overall, a significantly wider output voltage range can be achieved, even with reduced power.

[0038] Figure 4 shows this transmission power reduction in the second operating state, starting from a duty cycle of approximately 0.5. The exact value at which power reduction begins, and the degree of reduction, are determined based on specific parameters of the DC / DC converter, ensuring that they do not exceed the allowable operating parameters of the components.

[0039] Therefore, at a given output voltage, the second operating state allows for an expansion of the input voltage range in which a given DC / DC converter can operate, without requiring any changes to the DC / DC converter hardware.

[0040] For example, if the output voltage is 14V, the input voltage range can be expanded from 200V~400V to 125V~400V.

[0041] [Explanation of symbols] 1 Primary side 2 Secondary side 3. Transformer 10 DC / DC Converters 11 Upper primary side DC terminal 12 Lower primary side DC terminal 19 Primary side bridge circuit 31 Upper secondary winding 32 Lower secondary winding 33 Upper transformer tap 34 Lower transformer tap 35. First upper switch 36. First lower switch 37. Second upper switch 38. Second lower switch 39 Center Tap 40 Secondary bridge circuit 41 Upper connecting rail 42 Lower connecting rail 43 Filter Inductance 44 filter capacity 45 Snubber capacity 47 Upper secondary side DC terminal 48 Lower secondary side DC terminal< / vct> < / vct>

Claims

1. A method for operating a DC / DC converter (10), The DC / DC converter (10) comprises a primary side (1), a secondary side (2), and a transformer (3) having a center tap (39). The upper transformer tap (33) and the lower transformer tap (34) are connected via a full bridge circuit to the upper terminal point of the snubber capacity (45) by the upper switches (35, 37) and to the lower terminal point of the snubber capacity (45) by the lower switches (36, 38). The lower secondary DC terminal (48) is connected to the lower terminal point of the snubber capacitance (45), and the upper secondary DC terminal (47) is connected to the center tap (39) via the filter inductance (43). The aforementioned method, - In the first operating state, the lower switches (36, 38) alternately apply positive voltage pulses of the secondary voltage between the center tap (39) and the lower secondary DC terminal (48) with a duty cycle d between 0 and 0.5, The process includes: during the voltage pulse, the lower switches (36, 38) are closed, and the upper switches (35, 37) redirect the current that would lead to the voltage peak when the lower switches are turned off to the snubber capacitance (45); - In the second operating state, the upper switches (35, 37) alternately apply positive voltage pulses of the secondary voltage between the center tap (39) and the upper terminal point of the snubber capacitance (45), The process involves closing the upper switches (35, 37) and opening the lower switches (36, 38) during the voltage pulse, Methods that include...

2. The method according to claim 1, A method in which, in the second operating state, following the application of a voltage pulse through one of the upper switches (35, 37), each of the upper switches (35, 37) is opened in order to limit the discharge of the snubber capacitance (45).

3. A method for operating a DC / DC converter (10), preferably according to claim 1 or 2, The DC / DC converter (10) comprises a primary side (1), a secondary side (2), and a transformer (3). - The transformer (3) is equipped with an upper transformer tap (33), a lower transformer tap (34), and a center tap (39) on the secondary side. - The secondary side (2) is equipped with a lower secondary DC terminal (48) and an upper secondary DC terminal (47), - The filter inductance (43) is connected between the upper secondary DC terminal (47) and the center tap (39). - The first upper switch (35) is connected between the upper transformer tap (33) and the first terminal point of the snubber capacity (45), where the second terminal point of the snubber capacity (45) is connected to the lower secondary DC terminal (48). - The second upper switch (37) is connected between the lower transformer tap (34) and the first terminal point of the snubber capacity (45), - The first lower switch (36) is connected between the upper transformer tap (33) and the lower secondary DC terminal (48), - The second lower switch (38) is connected between the lower transformer tap (34) and the lower secondary DC terminal (48). Each of the switches (35, 36, 37, 38) is equipped with a freewheeling diode. In the extended operating state, the DC / DC converter (10) is in the following state, namely, - First state in which the primary side of the transformer (3) is currentless, the first upper switch (35) and the second upper switch (37) are closed, and the first lower switch (36) and the second lower switch (38) are open; - A second state in which the transformer (3) generates a positive voltage at the upper transformer tap (33) relative to the lower transformer tap (34), the second upper switch (37) is open, and the second lower switch (38) is closed; - An optional third state in which the first upper switch (35) is open; - A fourth state in which the transformer (3) is switched to no current on the primary side, the first upper switch (35) and the second upper switch (37) are closed, and the second lower switch (38) is open; - A fifth state in which the transformer (3) generates a negative voltage at the upper transformer tap (33) relative to the lower transformer tap (34), the first upper switch (35) is open, and the first lower switch (36) is closed; - A sixth optional state in which the second upper switch (37) is open. A method characterized by repeatedly performing the following in this order.

4. The method according to claim 3, The duration during which the first state and / or the fourth state is active is [Math 1] And, d is the duty cycle of the DC / DC converter (10), and 0.5 < d < 1, f ｓｗ A method in which the switching frequency is the frequency at which the cycle is repeated across the six states.

5. The method according to claim 3 or 4, The duration during which the second state and / or the fifth state is active is 【Number 2】 And, n is the turns ratio of the primary winding to each secondary winding, and L ｌｐ L is the leakage inductance of the primary winding. ｌｓ１ and L ｌｓ２ This is the leakage inductance of the secondary winding, and C ｓｎｕｂｂｅｒ The method wherein the capacitance is the snubber capacitance (45).

6. A method according to any one of claims 1 to 5, A method for reducing transmission power when a duty cycle greater than 0.5 is applied.

7. The method according to claim 6, A method by which the transmitted power is reduced by reducing the output current.

8. A control unit for a DC / DC converter, which is programmed to perform the method according to any one of claims 1 to 7.

9. A DC / DC converter comprising the control unit described in claim 8.

10. A DC / DC converter for performing the method of any one of claims 1 to 7, excluding claim 2, A DC / DC converter in which the snubber capacity (45) is designed to be sufficiently large so as to eliminate the need for a switch-off procedure required to prevent the current of the snubber capacity (45) from oscillating and the snubber capacity (45) from discharging.

Images