Primary vs Secondary Side#
Understanding the distinction between primary and secondary sides of the DC/DC converter is essential for effective troubleshooting.
Overview#
The transformer (T901) divides the power supply into two electrically isolated sections:
Primary Side#
The primary side is everything connected to the battery input, before the transformer.
Components#
What Happens on the Primary Side#
- Battery power enters through fuses F901/F902 (protection)
- Input filter (L920, C905/C906) smooths the DC and reduces noise
- IC920 generates PWM at ~150kHz with dead-time control
- Q901/Q902 amplify the PWM signals to drive FET gates
- Q903-Q906 switch alternately, converting DC to high-frequency AC
- AC current flows through the transformer primary winding
Primary Side Characteristics#
| Parameter | Value |
|---|---|
| Input Voltage | 12-14.4V DC |
| Maximum Current | ~18A at full power |
| Switching Frequency | ~100-200kHz |
| FET Voltage (Vds) | 60V rating |
Secondary Side#
The secondary side is everything after the transformer, providing power to the amplifier stages.
Components#
What Happens on the Secondary Side#
- Transformer induces AC in secondary windings (magnetically coupled)
- Voltage is stepped up by the turns ratio (~1.6× from 5:8)
- Rectifier diodes convert AC back to pulsing DC
- Filter capacitors smooth to clean DC
- Linear regulators create stable ±14V for preamp
Secondary Side Characteristics#
| Rail | Voltage | Current | Powers |
|---|---|---|---|
| ±25V | ±25V DC | High | Output transistors |
| ±23V | ±23V DC | Medium | Driver transistors |
| ±14V | ±14V DC | Low | Preamp op-amps |
The Isolation Barrier#
The transformer provides galvanic isolation - there is no direct electrical connection between primary and secondary.
Benefits of Isolation#
- Voltage transformation - Step up 14V to ±25V efficiently
- Ground isolation - Secondary can have independent ground reference
- Noise rejection - Switching noise partially blocked
- Safety - Fault on one side doesn’t directly affect the other
Fault Reflection: The Critical Concept#
A fault on the secondary side reflects back to the primary as increased load.
Why This Matters for Diagnosis#
| If Fault Is On… | You’ll See… | But the Cause Is… |
|---|---|---|
| Secondary | FETs overheating | Shorted diode/transistor downstream |
| Secondary | Fuses blowing | Short on ±25V or ±23V rail |
| Primary | Same symptoms | Bad FETs or gate drivers |
This is why you can’t just replace FETs - if a secondary fault caused the original failure, it will kill the new FETs too.
Diagnosis Strategy#
Your Symptom Analysis#
Your observation: Voltage clamped at 8V, current rising to 1.2A
Most likely cause: Q901/Q902 gate drivers not providing enough drive, causing FETs to operate in linear region (high resistance, high power dissipation).
Test priority:
- Q901/Q902 (primary side gate drivers)
- D801-D803, D808 (secondary side rectifiers)
- Secondary rail resistance to ground
- Transformer winding resistance