Startup Circuit Using Emitter-Switched Current Source
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Solution Overview
Problem
Switching power supplies face challenges in providing initial energy to control circuitry when operating voltage limits are below the input voltage range, leading to high power dissipation and no-load power losses, especially in DC/DC converters and AC/DC power supplies with varying input voltages.
Innovation Solution
A startup circuit using an emitter-switched current source with a depletion-mode J-FET and a bipolar transistor, coupled with a resistive divider network, generates a startup voltage for the control circuitry, enabling efficient charging of a storage capacitor and reducing power dissipation by disabling the startup current source once the power converter is operational.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If a resistor or current source is used to charge the startup capacitor from the input source, then the control circuitry can be enabled when the capacitor voltage reaches the operating voltage, but high power dissipation occurs during normal operation irrespective of output loading
Solution Approach 1:
The patent applies a dynamic approach by using a depletion-mode J-FET as a switch that can be turned on or off based on operating conditions. The J-FET is normally on during startup to charge the capacitor, and can be turned off during normal operation to eliminate power dissipation. This dynamic switching state resolves the contradiction between ensuring reliable startup and minimizing ongoing energy loss.
Solution Approach 2:
The invention changes the resistance parameter dynamically by using a J-FET whose resistance can be controlled from low (during startup) to high (during normal operation). The gate voltage control allows the resistance to be adjusted based on whether the power supply is in startup or normal operation mode, thereby resolving the power dissipation issue while maintaining startup capability.
2Reliability
If the startup current source is sized to provide sufficient current at minimum input voltage, then startup can be achieved, but much higher power dissipation occurs at larger maximum input voltage during normal operation
Solution Approach 1:
The J-FET switch provides dynamic control over the startup current source, allowing it to be active only when needed during startup. Once the power supply is operating normally and the auxiliary power source is available, the J-FET turns off the startup current source, preventing excessive power dissipation at higher input voltages during normal operation.
Solution Approach 2:
The startup current source operates periodically - active during the startup phase and inactive during normal operation. The J-FET-controlled switching creates this periodic action pattern, enabling the system to achieve reliable startup while avoiding continuous power dissipation that would occur with a always-on current source.
3Reliability
If the startup circuit remains active during steady state operation, then the startup capacitor remains charged, but continuous power dissipation occurs irrespective of whether the converter is online or in standby
Solution Approach 1:
The patent extracts the startup current source from the continuously active circuitry by placing it under J-FET control. The J-FET allows the startup current source to be disconnected (taken out of the active circuit) during normal operation, eliminating unnecessary power dissipation while the auxiliary power source continues to maintain the startup capacitor voltage for control circuitry operation.
Solution Approach 2:
The system uses its own auxiliary power source to maintain the startup capacitor voltage during normal operation, making the startup circuit self-sustaining without requiring continuous power from the input source. The J-FET-controlled startup current source provides initial charging, then the auxiliary winding takes over to maintain voltage, eliminating continuous power dissipation from the input side.
Applied Scientific Principles
This section explains which scientific principles are used to turn an abstract innovation direction into a practical engineering solution.
Function Achieved in This Case
The solution reduces no-load power loss and ensures efficient startup by automatically providing charging current only when necessary, disabling the startup circuit to minimize power dissipation during steady-state operation, thus enhancing the efficiency of the power supply.
Implementation Method 1
an emitter-switched current source (Q2, Q3) having a normally on switching transistor (Q3) that conducts an enabling current (Ic(Q2)) in the absence of the inhibitory control signal
Implementation Method 2
the bipolar transistor conducting the enabling current as collector current when the normally on transistor is also conducting
Implementation Method 3
a storage capacitor (Caux) coupled to a power magnetic element (T1) to maintain a steady state value of an auxiliary voltage (Vaux)
Implementation Method 4
switching and control circuitry configured to couple energy from the input-side DC voltage to the power magnetic element during the steady state operation
Data Source
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AI summary
A power converter startup circuit establishes an operating voltage for control circuitry during startup and is then disabled to reduce no-load power dissipation. The startup circuit has a normally on characteristic to automatically provide startup charging current for a startup capacitor. The control circuitry begins operating as the startup capacitor voltage reaches an operating value, and it generates an inhibitory signal that disables the startup circuit to stop the startup charging current and reduce power dissipation. The normally on characteristic is achieved by an emitter switched current source employing a normally on device such as a depletion-mode J-FET. A resistor divider network provides both biasing for the startup current source and a point of monitoring the power supply input voltage during steady state operation.