Synchronous buck converter with output current sensing

• US 6,781,354 B2
• Filed: 04/03/2003
• Issued: 08/24/2004
• Est. Priority Date: 04/03/2002
• Status: Expired due to Term

• Current Assignee: Infineon Technologies Americas Corp. (Infineon Technologies AG)
• Original Assignee: International Rectifier Corporation (Infineon Technologies AG)
• Inventors: Zhang, Jason
• Primary Examiner(s): May, Shawn Riley
• Application Number:
  US10/406,683
• Publication Number:
  US 20030210020A1
• Time in Patent Office: 509 Days
• Field of Search: 323/224, 323/284, 323/283, 323/282
• US Class Current: 323/224
• CPC Class Codes: H02M 3/1584 with a plurality of power processing stages connected in parallel
• CPC Class Codes: H02M 3/1588 comprising at least one synchronous rectifier element H02M3/1582, H02M3/1584 take precedence
• 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 synchronous buck converter manufacturable as an MCM having an output current sensing circuit. The device employs MOSFETS as the switching transistors, and a sensing circuit including a sampling switch operated synchronously on and off with the shunt MOSFET, a variable gain amplifier, the sampling switch being coupled to provide a signal to the variable gain amplifier representing the voltage across the R_DS-ON of the shunt MOSFET, and a circuit, e.g., an RC circuit, associated with the variable gain amplifier implementing a low pass filter whereby the output of the variable gain amplifier is substantially independent of the value of the inductor and the magnitude of any time varying component of the signal input to the sampling switch. When the device is packaged an MCM, the current sense circuit gain can be trimmed based on the R_DS-ON value. The current sense gain can also be adjusted according to the module temperature by using temperature sensitive devices inside the IC to eliminate the R_DS-ON temperature variation, and according to the gate voltage to eliminate the R_DS-ON variation due to gate voltage changes.

Claims

1. A synchronous buck converter comprising:

a first switching transistor connected between an input node and a first node;

a second switching transistor connected between the first node and a second node;

a series inductor connected between the first or second node and an output node;

a capacitor connected between the output node and the second node;

a sensing circuit operative to generate a signal representative of the output current of the converter, the sensing circuit being comprised of;

a sampling switch operated synchronously on and off with one of the switching transistors,

a variable gain amplifier,

the sampling switch being coupled to provide a signal to the variable gain amplifier representing the voltage across the one switching transistor when it is fully conducting, and

a circuit associated with the variable gain amplifier implementing a low pass filter whereby the output of the variable gain amplifier is substantially independent of the inductance of the inductor and the magnitude of any time varying component of the signal input to the sampling switch; and

a drive circuit which is operative to turn the first and second switching transistors on and off according to a variable duty cycle determined by the difference between the voltage output of the sensing circuit and a reference voltage.

2. A device as described in claim 1, wherein the low pass filter is implemented by an RC circuit connected between the one switching transistor and an input terminal of the variable gain amplifier.

3. A device as described in claim 2, wherein:

the sampling switch is operated synchronously on and off with the second switching transistor; and

the low pass filter is connected to the second node, whereby the sample represents the voltage across the second switching transistor.

4. A device as described in claim 2, wherein:

the sampling switch is operated synchronously on and off with the first switching transistor; and

the low pass filter is connected to the input node, whereby the sample represents the voltage across the first switching transistor.

5. A device as described in claim 1, wherein the low pass filter is implemented by selection of the gain bandwidth of the variable gain amplifier.

6. A device as described in claim 1, wherein the sampling switch is operated synchronously on and off with the second switching transistor.

7. A device as described in claim 1, wherein the sampling switch is operated synchronously on and off with the first switching transistor.

8. A device as described in claim 1, wherein the sampling switch is operated by the drive circuit.

9. A device as described in claim 1, wherein the first and second switching transistors are MOSFETS.

10. A device as described in claim 1, wherein the entire device except for the series inductor and the output capacitor is included in a multi-chip module.

11. A device as described in claim 1, wherein the entire device is included in a multi-chip module.

12. A device as described in claim 1, wherein the sensing circuit is packaged in a module separate from the remainder of the device.

13. A device as described in claim 1, wherein the variable gain amplifier is a transconductance amplifier.

14. A device as described in claim 1, further including a delay circuit operative to provide predetermined small delay between the time the one switching transistor is turned on and the time the sampling switch is turned on.

15. A device as described in claim 1, wherein:

the sensing circuit, the drive circuit, and the first and second switching transistors are assembled as a multi-chip module; and

the device further includes a temperature sensitive device within the multi-chip module operative to vary the gain of the variable gain amplifier to compensate for temperature dependent changes in resistance of the current path of the one switching transistor when it is in the conducting state.

16. A device as described in claim 15, wherein the temperature sensitive device is a diode.

17. A device as described in claim 15, wherein:

the switching transistors are MOSFETS; and

the temperature sensitive device is operative to vary the gain of the variable gain amplifier in proportion to temperature dependent changes in the R_DS-ON of the one MOSFET.

18. A device as described in claim 1, wherein the sensing circuit, the drive circuit, and the first and second switching transistors are assembled as a multi-chip module;

and

the device further includes a voltage sensitive device within the multi-chip module operative to vary the gain of the variable gain amplifier to compensate for changes in resistance of the current path of the one switching transistor when it is in the conducting state resulting from variations in the voltage of a control signal provided by the drive circuit.

19. A device as described in claim 1, wherein the gain of the variable gain amplifier is set to provide a predetermined output signal for a predetermined current flowing through the current path of the one switching transistor when it is in the conducting state to compensate for variations in resistance of the current path of the one switching transistor when it is in the conducting state resulting from component to component variations.

20. A device as described in claim 1, wherein:

the first and second switching transistors are MOSFETS; and

the gain of the variable gain amplifier is set to provide a predetermined output signal for a predetermined current flowing through the channel of the one MOSFET to compensate for variations in R_DS-ON of the one MOSFET resulting from component to component variations.