Control circuit for synchronous rectifiers in DC/DC converters to reduce body diode conduction losses

• US 6,535,400 B2
• Filed: 12/19/2001
• Issued: 03/18/2003
• Est. Priority Date: 03/30/2001
• Status: Active Grant

• Current Assignee: Texas Instruments, Inc.
• Original Assignee: Texas Instruments, Inc.
• Inventors: Bridge, Christopher David
• Primary Examiner(s): BERHANE, ADOLF D
• Application Number:
  US10/024,870
• Publication Number:
  US 20020141209A1
• Time in Patent Office: 454 Days
• Field of Search: 363/20, 363/21.01, 363/21.04, 363/21.05, 363/21.06, 363/44, 363/52, 363/84, 363/89, 363/127
• US Class Current: 363/21.06
• CPC Class Codes: H02M 1/0048 Circuits or arrangements for reducing losses using snubbers H02M1/34
• CPC Class Codes: H02M 3/33592 having a synchronous rectifier circuit or a synchronous freewheeling circuit at the secondary side of an isolation transformer
• CPC Class Codes: Y02B 70/10 Technologies improving the efficiency by using switched-mode power supplies [SMPS], i.e. efficient power electronics conversion e.g. power factor correction or reduction of losses in power supplies or efficient standby modes

Abstract

A controller for a switching power supply having first and second synchronous rectifiers that minimizes the reverse recovery time and body diode conduction losses of each of the synchronous rectifiers. The controller includes a first controller that predicts the optimal turn-on and turn-off time of the first synchronous rectifier as a function of voltage measurements of the previous switching cycle and the timing of the pulse width modulator signal in the current switching cycle. The controller also includes a second controller that predicts the optimal turn-on and turn-off time of the second synchronous rectifier as a function of voltage measurements of the previous switching cycle and the timing of the pulse width modulator signal in the current switching cycle.

Claims

1. A control system for a switched mode power supply including first and second synchronous rectifiers, each synchronous rectifier including a parasitic body diode, and having a first terminal, a second terminal, and a control terminal, the switched mode power supply further including a pulse width modulator (PWM) providing a plurality of PWM signal pulses, each PWM signal pulse corresponding to a switching cycle, the control system comprising:

a first control module providing a first control signal having a turn-on portion and a turn-off portion to the control terminal of the first synchronous rectifier, the first control module receiving as input signals a first measurement signal indicative of the magnitude of the voltage between the first and second terminals of the first synchronous rectifier, a second measurement signal indicative of the magnitude of the first control signal, and a PWM signal pulse corresponding to the current switching cycle, the first control module configured and arranged to predict the optimal turn-on time of the first synchronous rectifier as a function of the first and second measurement signals of the previous switching cycle, and the pulse signal of the current switching cycle and to configure the first control signal to provide the turn-on portion of the first control signal to the control terminal of the first synchronous rectifier accordingly, the first control module further configured and arranged to predict the optimal turn-off time for the first synchronous rectifier as a function of the first measurement signal of the previous switching cycle, and the PWM pulse signal of the current switching cycle and to configure the first control signal to provide the turn-off portion of the first control signal to the control terminal of the first synchronous rectifier wherein the conduction of the parasitic body diode of the first synchronous rectifier is substantially minimized.

2. The control system of claim 1 wherein the first control module includes:

a first variable delay module receiving as an input the PWM signal pulse, the first delay module configured and arranged to provide as an output a plurality of delayed PWM signal pulses;

a first turn-on module receiving as a first input the first measurement signal and receiving as a second input the second measurement signal, the first turn on module coupled to the delay module, and the first turn-on module configured and arranged to predict the turn-on delay such that the first synchronous rectifier is turned on as the first measurement signal equals zero, the first turn-on module providing the first variable delay module with a first turn-on selection signal to select one of the plurality of delayed PWM signal pulses to provide the selected PWM signal pulse as the first turn-off portion of the first control signal to turn the first synchronous rectifier on at the predicted time, wherein the body diode conduction of the first synchronous rectifier is minimized;

a first turn-off module receiving as an input the first measurement signal, the first turn-off module coupled to the first delay module, the first turn-off module configured and arranged to predict the turn-off time of the first synchronous rectifier such that the first synchronous rectifier is turned off as the first measurement signal drops below the first predetermined threshold, the first turn-off module providing the first variable delay module with a first turn-off selection signal to select one of the plurality of delayed PWM signal pulses as the turn-off portion of the first control signal to turn the first synchronous rectifier off at the predicted time, wherein the body diode conduction of the first synchronous rectifier is minimized.

3. The control system of claim 2 wherein the first turn-on module includes:

a first turn-on threshold detector having first and second inputs coupled to the first control signal and to the first measurement signal and configured and arranged to detect when the first and second inputs are above a first predetermined turn-on threshold, the first threshold detector providing as an output a first turn-on threshold signal;

a first latch coupled to the output of the first threshold detector, the first latch operative to provide a latched first threshold output;

a turn-on counter module providing a first turn-on output count signal, the first counter receiving the latched first turn-on threshold output count signal and responsive to the latched first turn-on threshold count output signal by incrementing and decrementing a turn-on output count signal appropriately;

a first turn-on multiplexer receiving the first turn-on output count signal from the first turn-on counter, the first turn-on multiplexer coupled to the first variable delay module and receiving the plurality of variable delayed PWM signals, the first turn-on multiplexer being responsive to the first turn-on output count signal by selecting one of the plurality of variable delayed PWM signals and providing the selected variable delayed PWM signal as the first turn-on portion of the first control signal.

4. The control system of claim 2 wherein the first turn-off module includes:

a first turn-off threshold detector having a first input coupled to the second threshold value and a second input coupled to the first measurement signal, the first turn-off threshold detector operative to detect when the second input has fallen below the second threshold value and to provide a first turn-off threshold output signal;

a latch coupled to the first turn-off threshold detector and operative to provide a latched first turn-off threshold output signal;

a first turn-off counter module providing a first turn-off output count signal, the first turn-off counter module receiving the latched first turn-off threshold output signal and responsive to the latched first turn-off threshold output signal by incrementing and decrementing a turn-off output count signal appropriately;

a first turn-off multiplexer receiving the first turn-off output count signal from the first turn-off counter, the turn-off multiplexer coupled to the first variable delay module and receiving the plurality of variable delayed PWM signals, the first turn-off multiplexer being responsive to the first turn-off output count signal by selecting one of the plurality of variable delayed PWM signals and providing the selected variable delayed module output as the first turn-off portion of the first control signal.

5. A control system for a switched mode power supply including first and second synchronous rectifiers, each synchronous rectifier including a parasitic body diode, and having a first terminal, a second terminal, and a control terminal, the switched mode power supply further including a pulse width modulator (PWM) providing a plurality of PWM signal pulses, each PWM signal pulse corresponding to a switching cycle, the control system comprising:

a first control module providing a first control signal having a turn-on portion and a turn-off portion to the control terminal of the first synchronous rectifier, the first control module receiving as input signals a first measurement signal indicative of the magnitude of the voltage between the first and second terminals of the first synchronous rectifier, a second measurement signal indicative of the magnitude of the first control signal, and a PWM signal pulse corresponding to the current switching cycle, the first control module configured and arranged to predict the optimal turn-on time of the first synchronous rectifier as a function of the first and second measurement signals of the previous switching cycle and the pulse signal of the current switching cycle, and to configure the first control signal to provide the turn-on portion of the first control signal to the control terminal of the first synchronous rectifier accordingly, the first control module further configured and arranged to predict the optimal turn-off time for the first synchronous rectifier as a function of the first measurement signal of the previous switching cycle and the PWM pulse signal of the current switching cycle, and to configure the first control signal to provide the turn-off portion of the first control signal to the control terminal of the first synchronous rectifier wherein the conduction of the parasitic body diode of the first synchronous rectifier is substantially minimized;

a second control module to provide a second control signal having a second turn-on portion and a second turn-off portion, the second control module providing the second control signal to the control electrode of the second synchronous rectifier, the second control module receiving as inputs a third measurement signal indicative of the magnitude of the voltage between the first and second terminals of the second synchronous rectifier, a fourth measurement signal indicative of the magnitude of the voltage of the second control signal, a PWM signal corresponding to the current switching cycle, and the first measurement signal, the second control module configured and arranged to predict the optimal turn-off time for the second synchronous rectifier and to configure the second control signal to provide the turn-off portion of the second control signal to the control terminal of the second synchronous rectifier as a function of the first and third measurement signals of the previous switching cycle and the pulse signal of the current switching cycle, the second control module further configured and arranged to detect when the second measurement signal has first increased above a first threshold level and has subsequently decreased to a value less than a second threshold level whereupon the second control module configures the second control signal to provide the turn-on portion of the second control signal to the control terminal of the second synchronous rectifier.

6. The control system of claim 5 wherein the first control module includes:

a first variable delay module receiving as an input the PWM signal pulse, the first delay module configured and arranged to provide as an output a plurality of delayed PWM signal pulses;

a first turn-on module receiving as a first input the first measurement signal and receiving as a second input the second measurement signal, the first turn on module coupled to the delay module, and the first turn-on module configured and arranged to predict the turn-on delay such that the first synchronous rectifier is turned on as the first measurement signal equals zero, the first turn-on module providing the first variable delay module with a first turn-on selection signal to select one of the plurality of delayed PWM signal pulses to provide the selected PWM signal pulse as the first turn-off portion of the first control signal to turn the first synchronous rectifier on at the predicted time, wherein the body diode conduction of the first synchronous rectifier is minimized;

a first turn-off module receiving as an input the first measurement signal, the first turn-off module coupled to the first delay module, the first turn-off module configured and arranged to predict the turn-off time of the first synchronous rectifier such that the first synchronous rectifier is turned off as the first measurement signal drops below the first predetermined threshold, the first turn-off module providing the first variable delay module with a first turn-off selection signal to select one of the plurality of delayed PWM signal pulses as the turn-off portion of the first control signal to turn the first synchronous rectifier off at the predicted time, wherein the body diode conduction of the first synchronous rectifier is minimized.

7. The control system of claim 6 wherein the first turn-on module includes:

a first turn-on threshold detector having first and second inputs coupled to the first control signal and to the first measurement signal and configured and arranged to detect when the first and second inputs are above a first predetermined turn-on threshold, the first threshold detector providing as an output a first turn-on threshold signal;

a first latch coupled to the output of the first threshold detector, the first latch operative to provide a latched first threshold output;

a turn-on counter module providing a first turn-on output count signal, the first counter receiving the latched first turn-on threshold output count signal and responsive to the latched first turn-on threshold count output signal by incrementing and decrementing a turn-on output count signal appropriately;

a first turn-on multiplexer receiving the first turn-on output count signal from the first turn-on counter, the first turn-on multiplexer coupled to the first variable delay module and receiving the plurality of variable delayed PWM signals, the first turn-on multiplexer being responsive to the first turn-on output count signal by selecting one of the plurality of variable delayed PWM signals and providing the selected variable delayed PWM signal as the first turn-on portion of the first control signal.

8. The control system of claim 6 wherein the first turn-off module includes:

a first turn-off threshold detector having a first input coupled to the second threshold value and a second input coupled to the first measurement signal, the first turn-off threshold detector operative to detect when the second input has fallen below the second threshold value and to provide a first turn-off threshold output signal;

a latch coupled to the first turn-off threshold detector and operative to provide a latched first turn-off threshold output signal;

a first turn-off counter module providing a first turn-off output count signal, the first turn-off counter module receiving the latched first turn-off threshold output signal and responsive to the latched first turn-off threshold output signal by incrementing and decrementing a turn-off output count signal appropriately;

a first turn-off multiplexer receiving the first turn-off output count signal from the first turn-off counter, the turn-off multiplexer coupled to the first variable delay module and receiving the plurality of variable delayed PWM signals, the first turn-off multiplexer being responsive to the first turn-off output count signal by selecting one of the plurality of variable delayed PWM signals and providing the selected variable delayed module output as the first turn-off portion of the first control signal.

9. The control system of claim 5 wherein the second control module includes:

a delay module receiving as an input the PWM pulse signals of the pulse width modulator providing as an output a plurality of variable delayed PWM pulse signals;

a turn-off module receiving as a first input the first control signal and receiving as a second input the first measurement signal,

the turn-off module configured and arranged to predict the turn-off time of the second synchronous rectifier such that the second synchronous rectifier is turned off as the first measurement signal is less than a first predetermined turn-off threshold, the turn-off module providing the second delay module with a delay selection signal to configure the delayed PWM pulse signal as the turn-off portion of the second control signal and to provide the turn-off portion of the second control signal to the control terminal of the second synchronous rectifier to turn the second synchronous rectifier off at the predicted time, wherein the body diode conduction of the second synchronous rectifier is minimized;

a turn-on module receiving as an input the third measurement signal, the second turn-on module coupled to the delay module, the turn-on module configured and arranged such that the second control signal turns on the second synchronous rectifier as the third measurement signal has risen above a first predetermined turn-on threshold and subsequently drops below a second predetermined turn-on threshold, the second turn-on module providing the turn-on portion of the second control signal to turn the second synchronous rectifier on at the predicted time, wherein the body diode conduction of the second synchronous rectifier is minimized.

10. The control system of claim 9 wherein the second turn-off module includes:

a turn-off threshold detector having a first input coupled to the second control signal and a second input coupled to the first measurement signal, the turn-off threshold detector operative to detect when the second control signal is below a predetermined third turn-off threshold and the first measurement signal is above the predetermined third turn-off threshold and to provide a second threshold output signal;

a turn-off latch coupled to the turn-off threshold detector and operative to provide a latched second threshold output signal;

a turn-off counter module providing a turn-off output count signal, the turn-off counter module receiving the latched second threshold output and responsive to the latched second threshold output by incrementing and decrementing a second turn-off output count signal appropriately;

a turn-off multiplexer receiving the second turn-off output count signal from the second turn-off counter, the turn-off multiplexer coupled to the variable delay module and receiving the plurality of variable delayed PWM signals, the turn-off multiplexer being responsive to the turn-off output count signal by selecting one of the plurality of variable delayed PWM signals and providing the selected variable delayed module output as the turn-off portion of the first control signal.

11. The control system of claim 9 wherein the turn-on module includes:

a second turn-on threshold detector having a first input coupled to a second turn-on threshold value, a second input coupled to the third measurement signal, and a third input coupled to a third turn-on threshold detector, the second turn-on threshold detector operative to detect when the second input has risen above the first turn-on threshold and subsequently fallen below the second turn-on threshold and to provide the turn-on portion of the second control signal.