Power Supply (DC/DC Converter)#
The power supply is the heart of the amplifier, converting 12-14.4V automotive power into the multiple voltage rails required by the amplifier stages.
Block Diagram#
Component Details#
PWM Controller: IC920 (uPC494)#
The uPC494 is a classic PWM controller IC that generates the switching signals for the DC/DC converter.
Key Functions:
- Generates two complementary PWM outputs (pins 8 and 11)
- Internal 5V reference (pin 14)
- Dead-time control to prevent shoot-through
- Error amplifier for voltage regulation feedback
Pin Functions:
| Pin | Function | Normal Voltage |
|---|---|---|
| 1 | Non-inv input | 1.6V (varies w/temp) |
| 2 | Non-inv input | 2.5V |
| 3 | Inv input | 0.1V |
| 4 | GND | 0V |
| 5 | Ct (timing cap) | 1.8V |
| 6 | Rt (timing res) | 3.8V |
| 7 | GND | 0V |
| 8 | Output A | Pulsing (to FETs) |
| 11 | Vcc | 14.4V |
| 12 | Vcc | 14.4V |
| 14 | Vref | 5.0V |
| 15 | Dead time | 2.5V |
| 16 | GND | 0V |
Gate Drivers: Q901, Q902 (2SB1132)#
These PNP transistors amplify the PWM signals from IC920 to drive the FET gates with sufficient current.
Why They Matter:
- FET gates require significant charge to switch quickly
- Weak gate drive = FETs stuck in linear mode = thermal runaway
- If damaged, FETs may partially conduct and overheat
Normal Voltages:
| Terminal | Q901 | Q902 |
|---|---|---|
| Emitter | 5.6V | 5.6V |
| Collector | 0V | 0V |
| Base | 6V | 6V |
Switching FETs: Q903-Q906 (2SK3662)#
Four N-channel MOSFETs in a push-pull configuration drive the transformer primary.
Specifications:
- Vds: 60V
- Id: 45A continuous
- Vgs(th): 2-4V
- Rds(on): ~14mΩ
Normal Voltages (running):
| Terminal | Voltage |
|---|---|
| Gate | 5.6V |
| Drain | 14.4V |
| Source | 0V |
Transformer: T901#
Center-tapped push-pull transformer with 5:8:1 ratio.
Windings:
- Primary: Connected to FET drains
- Secondary 1: Feeds ±25V rectifiers
- Secondary 2: Feeds ±23V rectifiers
Secondary Rectifiers#
| Diode | Part | Rail | Type |
|---|---|---|---|
| D802 | FCH10A15 | +25V | Fast recovery |
| D803 | FRH10A15 | -25V | Fast recovery |
| D801 | 11EFS2 | +23V | Fast recovery |
| D808 | 11EFS2 | -23V | Fast recovery |
Voltage Regulators#
| Transistor | Part | Output | Type |
|---|---|---|---|
| Q801 | 2SC3421 | +14V | NPN linear regulator |
| Q802 | 2SA1358 | -14V | PNP linear regulator |
Common Failure Modes#
1. FET Failure (Q903-Q906)#
Cause: Overcurrent, shoot-through, or downstream short Symptom: No output, blown fuses, possible smoke Note: FETs rarely fail alone - always check root cause
2. Gate Driver Failure (Q901/Q902)#
Cause: Often fails with or causes FET failure Symptom: FETs run in linear mode, overheat Test: Check transistor junctions with diode mode
3. Rectifier Diode Failure (D801-D803, D808)#
Cause: Overcurrent from output stage failure Symptom: Dead short on secondary, heavy primary current Test: Check forward/reverse with diode mode
4. Transformer Failure (T901)#
Cause: Shorted turns from overcurrent Symptom: Heavy primary current, possible overheating Test: Measure winding resistance, check for shorts between windings
Diagnostic Entry Points#
When troubleshooting the power supply:
- Start with IC920 - Verify 5V Vref and PWM pulses
- Check gate drivers - Q901/Q902 junction tests
- Test rectifiers - Forward/reverse diode tests
- Measure rail loads - Resistance from each rail to GND
- Verify transformer - Winding resistance and isolation