# US 8,541,987 B2 · Low loss discharge circuits for EMI filter capacitors

> Agents: this data is available as JSON from the patentgrep API and MCP server — see https://patentgrep.com/llms.txt

- Number: US 8,541,987 B2
- Document ID: US8541987B2
- Status: Granted
- Kind: B2
- Application number: 12/956,351
- Filed: 2010-11-30
- Published: 2012-01-12
- Granted: 2013-09-24
- Earliest claimed: 2010-07-07
- Assignees: Monolithic Power Systems, Inc.
- Applicants: Donald R. Disney; Michael Hsing; James Nguyen; James Moyer
- Inventors: Donald R. Disney; Michael Hsing; James Nguyen; James Moyer
- Web page: https://patentgrep.com/patent/US8541987B2

## CPC

- H02M 1/44: Circuits or arrangements for compensating for electromagnetic interference in converters or inverters
- H02M 1/32: Means for protecting converters other than automatic disconnection
- H02M 1/322: {Means for rapidly discharging a capacitor of the converter for protecting electrical components or for preventing electrical shock}
- H02M: APPARATUS FOR CONVERSION BETWEEN AC AND AC, BETWEEN AC AND DC, OR BETWEEN DC AND DC, AND FOR USE WITH MAINS OR SIMILAR POWER SUPPLY SYSTEMS; CONVERSION OF DC OR AC INPUT POWER INTO SURGE OUTPUT POWER; CONTROL OR REGULATION THEREOF (transformers H01F; dynamo-electric converters H02K47/00; controlling transformers, reactors or choke coils, control or regulation of electric motors, generators or dynamo-electric converters H02P)

## Abstract

A discharge circuit for an EMI filter capacitor includes normally-ON transistors. The normally-ON transistors may be controlled to limit current through them when an AC source is coupled across the discharge circuit. When the AC source is disconnected from the discharge circuit, the normally-ON transistors turn ON to allow current flow through them. The current flow allows the EMI filter capacitor to be discharged by a discharge resistor.

## Claims

### Claim 1

A discharge circuit for a capacitor, the discharge circuit comprising:
a first normally-ON transistor; and
a second normally-ON transistor, a source of the first normally-ON transistor being coupled to a source of the second normally-ON transistor, a gate of the first normally-ON transistor being coupled to a gate of the second normally-ON transistor, the first and second normally-ON transistors being configured to allow current flow through a drain of the first normally-ON transistor and a drain of the second normally-ON transistor when an AC source coupled across the discharge circuit is disconnected from the discharge circuit, the current flow allowing the capacitor of an AC-DC power supply to be discharged.

### Claim 2 (depends on 1)

The discharge circuit of claim 1 further comprising:
a first discharge resistor coupled to the drain of the first normally ON transistor and a second discharge resistor coupled to the drain of the second normally-ON transistor.

### Claim 3 (depends on 2)

The discharge circuit of claim 2 wherein the AC source is coupled in parallel with a series combination of the first discharge resistor, the second discharge resistor, and the discharge circuit.

### Claim 4 (depends on 1)

The discharge circuit of claim 1 wherein the first and second normally-ON transistors comprise JFETs.

### Claim 5 (depends on 1)

The discharge circuit of claim 1 wherein the first and second normally-ON transistors comprise depletion-mode MOSFETs.

### Claim 6

A method of discharging an EMI filter capacitor in an AC-DC power supply, the method comprising:
turning-ON a first normally-ON transistor and a second normally-ON transistor to allow current to flow through the first and second normally-ON transistors when an AC source coupled across a discharge circuit is disconnected; and
discharging an EMI filter capacitor of an AC-DC power supply when the current flows through the first and second normally-ON transistors in response to the disconnection of the AC source from the discharge circuit.

### Claim 7 (depends on 6)

The method of claim 6 wherein a first discharge resistor is coupled between the first normally ON transistor and one end of the AC source and a second discharge resistor is coupled between the second normally ON transistor and another end of the AC source.

### Claim 8 (depends on 6)

The method of claim 6 wherein the first and second normally-ON transistors comprise JFETs.

### Claim 9 (depends on 6)

The method of claim 6 wherein the first and second normally-ON transistors comprise depletion-mode MOSFETs.

### Claim 10

A discharge circuit for an EMI filter capacitor of an AC-DC power supply, the discharge circuit comprising:
a first capacitor;
a first normally-ON transistor; and
a second normally-ON transistor, a source of the first normally-ON transistor being coupled to a drain of the second normally-ON transistor, a gate of the first normally-ON transistor being coupled to a gate of the second normally-ON transistor, the second normally-ON transistor being configured to be turned OFF by charge on the first capacitor when an AC source is coupled across the discharge circuit, the first and second normally-ON transistors being configured to allow a current to flow through the first and second normally-ON transistors when the AC source is disconnected from the discharge circuit, the current allowing the EMI filter capacitor to be discharged.

### Claim 11 (depends on 10)

The discharge circuit of claim 10 further comprising:
a second capacitor configured to develop a voltage on a common node of the discharge circuit to charge the first capacitor when the AC source is coupled across the discharge circuit.

### Claim 12 (depends on 11)

The discharge circuit of claim 11 wherein the second capacitor is coupled to the source of the first normally-ON transistor and to the drain of the second normally-ON transistor.

### Claim 13 (depends on 11)

The discharge circuit of claim 11 wherein the second capacitor is coupled directly to the AC source.

### Claim 14 (depends on 11)

The discharge circuit of claim 11 wherein the second capacitor is coupled to the AC source through a discharge resistor.

### Claim 15 (depends on 10)

The discharge circuit of claim 10 further comprising:
a resistor coupled across the first capacitor, the resistor being configured to discharge the first capacitor when the AC source is disconnected from the discharge circuit.

### Claim 16 (depends on 10)

The discharge circuit of claim 10 further comprising a first discharge resistor coupled between a drain of the first normally ON transistor and the AC source.

### Claim 17 (depends on 16)

The discharge circuit of claim 16 further comprising a second discharge resistor, the first discharge resistor being coupled to one end of the AC source and the second discharge resistor being coupled to another end of the AC source.

### Claim 18 (depends on 10)

The discharge circuit of claim 10 further comprising:
a third normally-ON transistor; and
a fourth normally-ON transistor, a source of the third normally-ON transistor being coupled to a drain of the fourth normally-ON transistor, a gate of the third normally-ON transistor being coupled to a gate of the fourth normally-ON transistor, the fourth normally-ON transistor being configured to be turned OFF by charge on the first capacitor when the AC source is coupled across the discharge circuit, the third and fourth normally-ON transistors being configured to allow current flow through the third and fourth normally-ON transistors when the AC source is disconnected from the discharge circuit, the current flow through the third and fourth normally-ON transistors when the AC source is disconnected from the discharge circuit allowing the EMI filter capacitor of the AC-DC power supply to be discharged.

### Claim 19 (depends on 10)

The discharge circuit of claim 10 wherein the first and second normally-ON transistors comprise JFETs.

### Claim 20 (depends on 10)

The discharge circuit of claim 10 wherein the first and second normally-ON transistors comprise depletion-mode MOSFETs.