Start-up circuit for flyback converter having secondary pulse width modulation

• US 6,456,511 B1
• Filed: 06/29/2000
• Issued: 09/24/2002
• Est. Priority Date: 02/17/2000
• Status: Expired due to Fees

• Current Assignee: LAPIS Semiconductor Co., Ltd. (ROHM Co., Ltd.)
• Original Assignee: Tyco Electronics Corporation (TE Connectivity Limited)
• Inventors: Wong, Chuck
• Primary Examiner(s): NGUYEN, MATTHEW VAN
• Application Number:
  US09/608,459
• Time in Patent Office: 817 Days
• Field of Search: 363/16, 363/20, 363/21.01, 363/21.12, 363/21.13, 363/49, 363/95, 363/97, 363/131
• US Class Current: 363/21.13
• CPC Class Codes: H02M 1/36 Means for starting or stopping converters
• CPC Class Codes: H02M 3/33523 with galvanic isolation between input and output of both the power stage and the feedback loop

Abstract

An output isolated, switching power supply has a transformer with a primary and two secondaries, an electronic switch in series with the primary, a first rectifier and filter on the first secondary to provide bias power during both startup and operating modes, and a second rectifier and filter on the second secondary to provide regulated output power. A resistor-capacitor network on the primary side provide an initial operating condition, such as a single control pulse, to the electronic switch which causes sufficient energy to be transferred through the first secondary to supply sufficient startup energy to operate a current control integrated circuit on the secondary side in a staged fashion. After the initial operating condition, the current control integrated circuit generates and applies a control signal to the electronic switch through an isolation circuit to cause the electronic switch to turn on and off in controlled fashion in order to deliver regulated power to the output of the supply. The low voltage, secondary side, current control integrated circuit provides a further aspect of the present invention.

Claims

1. A switching power supply for translating alternating current of a voltage source within a standard range of voltage supply mains into regulated power supplying a load, the regulated power being electrically isolated from the voltage supply mains, the power supply comprising:

first rectifier means for rectifying the alternating current into a first rectified current, when connected to the voltage supply mains, and having a positive output node and a return node,

a series network including a primary winding of a first energy storing transformer means and a controlled electronic switching means connected across the positive output node and the return node,

the first energy storing transformer means also having at least one secondary winding electrically isolated from the primary winding,

the controlled electronic switching means having a control electrode,

first network means connected to the control electrode,

second rectifier means connected to the secondary winding for rectifying alternating energy stored in the first energy storing transformer means into a first secondary voltage,

first capacitor means connected to store said first secondary voltage during an initial startup interval,

control circuit means operated by the first secondary voltage and responsive to a control signal responsive to power supplied to the load for generating and putting out control pulses following the initial startup interval, and

an isolation circuit for transferring the control pulses to the control electrode for controlling duty cycle of the controlled electronic switching means in relation to the control signal.

2. The switching power supply set forth in claim 1 wherein the first network means comprises second capacitor means connected from the positive output node to the control electrode, and first resistor means connected from the control electrode to the return node.

3. The switching power supply set forth in claim 1 wherein the first network means comprises a self-resonant circuit including a first inductor means and a second capacitor means connected to cause the controlled electronic switching means to self-oscillate during an initial start-up interval and to stop self-oscillating following said initial start-up interval.

4. The switching power supply set forth in claim 3 wherein the first inductor means comprises a secondary winding of a pulse transformer within the isolation circuit.

5. The switching power supply set forth in claim 1 wherein the controlled electronic switching means comprises an enhancement mode power insulated gate metal-oxide-silicon field effect transistor (MOSFET) having a source electrode, a drain electrode, and a gate electrode, said series network including the source electrode and the drain electrode and the control electrode comprising the gate electrode.

6. The switching power supply set forth in claim 2 wherein the controlled electronic switching means comprises an enhancement mode power insulated gate metal-oxide-silicon field effect transistor (MOSFET) having a source electrode, a drain electrode, and a gate electrode, said series network including the source electrode and the drain electrode and the control electrode comprising the gate electrode, and wherein the second capacitor means and the first resistor means cause the MOSFET to conduct when power is first applied from the mains to the power supply for a period of time determined by a time constant established by respective values of capacitance and resistance of the first capacitor means and the first resistor means.

7. The switching power supply set forth in claim 6 wherein after initial conduction of the MOSFET ends, sufficient energy is transferred from the first transformer means and through the second rectifier means and stored in the second capacitor means to enable the control circuit to begin generating said control pulses under non-excessive operating power conditions at the load.

8. The switching power supply set forth in claim 1 wherein said first energy storing transformer means has a second secondary winding and further comprising third rectifier means for producing a second secondary voltage, and a current-limiting network comprising a third capacitor means, a second inductor means, and a fourth smoothing capacitor means for initially isolating said load from said second secondary winding during the initial startup mode and for thereafter filtering and providing said second secondary voltage as regulated DC power to said load.

9. The switching power supply set forth in claim 8 wherein the control circuit means includes DC voltage sensing means responsive to level of said second secondary voltage for providing said control signal.

10. The switching power supply set forth in claim 8 wherein the control circuit means includes output current sensing means responsive to a level of current flow through said second secondary winding for providing said control signal.

11. The switching power supply set forth in claim 10 wherein the output current sensing means includes trickle current switching means for enabling sensing of a plurality current flows through said secondary winding including a trickle current flow.

12. The switching power supply set forth in claim 8 wherein the control circuit means includes a maximum output current sensing and limiting means for limiting said control signal so that no more than a predetermined maximum output current flows to said load.

13. The switching power supply set forth in claim 1 wherein said first capacitor means has a relatively small capacitance value, further comprising third capacitor means having a relatively larger capacitance value, and wherein said control circuit means includes linear switching means for linearly switching the third capacitor means into a parallel connection with the first capacitor means when the control circuit has begun to generate said control pulses during the startup sequence.

14. The switching power supply set forth in claim 1 wherein the isolation circuit includes a pulse transformer having a primary winding connected to said control electrode and a secondary winding connected to said control circuit means.

15. The switching power supply set forth in claim 1 wherein said control circuit means is formed as a low voltage monolithic integrated circuit chip.

16. The switching power supply set forth in claim 1 comprising a lithium ion cell charger.

17. An isolated-output switching power supply comprising a transformer having a primary winding and a secondary winding, a first rectifier for rectifying input power from AC mains, a series network including the primary winding and a source-drain path of a switching field effect transistor, a first resistor-capacitor network connected to apply a declining voltage level derived from the rectified input power directly to a gate of the transistor at an initial power-on event so that the transistor conducts and transfers input power through the primary and into a core of the transformer until a time constant of the resistor-capacitor network causes the transistor to stop conduction, whereupon energy stored in the core of the transformer is transferred to said secondary winding, a second rectifier and small value smoothing capacitor connected to said secondary winding to produce an initial operating low voltage, and an integrated control circuit chip electrically connected to receive and use said initial operating low voltage to begin generating and putting out switching pulses, and isolation circuit means for transferring said switching pulses to the gate of the transistor after the transistor has stopped conduction following said declining voltage level.

18. The isolated-output switching power supply set forth in claim 17 wherein the transformer has a second secondary winding and further comprising a third rectifier for producing a second secondary voltage, and a current-limiting network comprising a third capacitor, a first inductor, and a fourth smoothing capacitor for initially isolating an output load of the power supply from the second secondary winding during an initial startup interval and for thereafter filtering and providing said second secondary voltage as regulated DC power to the load.

19. The isolated-output switching power supply set forth in claim 18 further comprising an output level monitor connected in a network including the second secondary winding and third rectifier, and wherein the integrated control circuit chip is electrically connected to the output level monitor and regulates duty cycle of the switching pulses in relation to monitored output level of the power supply flowing to the load.

20. An isolated-output switching power supply comprising a transformer having a primary winding and a secondary winding, a first rectifier for rectifying input power from AC mains, a series network including the primary winding and a source-drain path of a switching field effect transistor, a resonant circuit network connected to a gate of the transistor to cause said transistor to self-oscillate during an initial power-on interval so that the transistor transfers input alternating current through the primary and into a core of the transformer, said energy stored in the core of the transformer thereupon being transferred to said secondary winding, a second rectifier and small value smoothing capacitor connected to said secondary winding to produce an initial operating low voltage, and an integrated control circuit chip electrically connected to receive and use said initial operating low voltage to begin generating and putting out switching pulses, and isolation circuit means forming a part of said resonant circuit network for transferring said switching pulses to the gate of the transistor and causing said transistor to stop self-oscillation following said initial power-on interval.

21. For use within a switching power supply having an input side isolated from an output side by a power transformer, the primary side including a primary winding of the power transformer, a first rectifier and filter for rectifying and smoothing alternating current from power mains to provide primary direct current, a MOSFET switch having a source and drain current path in series with the primary winding and having a gate circuit, starting circuit means for causing the MOSFET switch to transfer energy through a core of the power transformer during an initial startup interval, the secondary side including a first secondary network having a first secondary winding and a second rectifier and filter for rectifying and smoothing said energy into a low level operating voltage, a low voltage current control integrated circuit for generating control pulses for controlling the gate circuit upon receiving the low level operating voltage, the secondary side further including a second secondary network having a second secondary winding and a third rectifier, isolator and filter for rectifying, initially isolating during the initial startup interval and then filtering and smoothing energy from the transformer into an output power for application to an external load, the integrated circuit comprising:

low level operating voltage monitoring means connected to monitor the level of operating voltage supplied from said first secondary network,

linear control means connected to add capacitance of an external capacitor to said second rectifier and filter as operating voltage level increases during the initial startup interval,

output power monitoring means for monitoring the output power for application to the external load, and

width-modulated pulse generation means for generating recurrent control pulses having widths controlled by monitored output power, for application through isolation circuit means to the gate of the MOSFET switch.

22. The integrated circuit set forth in claim 21 wherein the output power monitoring means includes voltage monitoring means and current monitoring means.

23. For use within a switching power supply having an input side isolated from an output side by a power transformer, the primary side including a primary winding of the power transformer, a first rectifier and filter for rectifying and smoothing alternating current from power mains to provide primary direct current, a MOSFET switch having a source and drain current path in series with the primary winding and having a gate circuit, starting circuit means for causing the MOSFET switch to transfer energy through a core of the power transformer during an initial startup interval, the secondary side including a first secondary network having a first secondary winding and a second rectifier and filter for rectifying and smoothing said energy into a low level operating voltage, a low voltage current control integrated circuit for generating control pulses for controlling the gate circuit upon receiving the low level operating voltage, the secondary side further including a second secondary network having a second secondary winding and a third rectifier, isolator and filter for rectifying, initially isolating during the initial startup interval and then filtering and smoothing energy from the transformer into an output power for application to an external load, the integrated circuit comprising:

output voltage-monitoring means for monitoring the output voltage for application to the external load to provide a voltage control,

output current monitoring means for monitoring the output current drawn by the external load to provide a current control,

internal switch means for switching between the voltage control and the current control,

pulse generation means for generating recurrent control pulses, comparator means responsive to the switch-selected voltage control or current control for putting out control pulses having widths controlled by monitored output voltage or current, for application through isolation circuit means to the gate of the MOSFET switch,

latched over-current sensing means for sensing secondary side overcurrent on a cycle-by-cycle basis and for inhibiting output of said control pulses when an overcurrent is sensed, and

reset means responsive to a power-on enable logical signal for resetting said latched over-current sensing means following inhibition of said control pulses.

24. The switching power supply set forth in claim 23 wherein the external load comprises a battery undergoing charging and wherein the output current monitoring means includes trickle current switching means for enabling sensing of a plurality current flows through said secondary winding including a trickle current flow through said battery.