Daewoo DV-F24S Service Manual - Section 3.

12

SECTION 3.

CIRCUIT OPERATION PRINCIPLES

3-1. POWER CIRCUIT

1. Basic Operation

The input voltage is rectified by the bridge diode (D801) and the capacitor (C807), ie, it is converted from AC to DC.

This voltage drives the gate of the FET (Q801) via the 'motive stage' (gate drive circuit) when the gate drive voltage exceeds

Vgs(th), it makes the FET turn on. However the mains input voltage can range from 110-220V ac, depending on country.

Therefore the voltage driving the FET'S gate can range correspondingly.

The circuit is designed to work for all input/output voltages, ie, Vgs>Vgs(th) for minimum input voltage.

When the FET is first turned on by the driver stage, the voltage in coil B is Vi(dc)

*

Nb/Np. This increases Vgs via C811,

increasing FET current and so increasing the energy stored in coil B. During a switching cycle, the FET is switched off by the

actions of voltage drop of components D803 and R806 combined with the switching control circuit.

When Q801 is switched off, the energy stored in the primary side is transmitted to the secondary side and so the output

voltage is generated. Once the power in the secondary (output) side starts dropping, this triggers the drive stage via the

back emf in the transformer and so the FET is switched on. At this time the negative voltage at C811 from the B coil is

rectified by D804 and the power is dissipated by R812. The transistor of opto-isolator IC801 is switched on drawing current

Ic away from the gate of FET Q801 and so switching the FET off.

Since the power transmitted to the secondary side is of high frequency, a fast recovery diode or Schottky barrier diode is

used for rectification. The voltage is smoothed using electrolytic capacitors which have a low impedence at the switching

frequency.

2. Snubber Circuit

The snubber circuit suppresses any sudden voltage rise when the FET turns on and keeps Vds (drain-source voltage of

FET) at a safe level. It also reduces noise in the FET.

In this snubber circuit (ie C808, D802, R807, C809, L804), once the FET is switched off, the energy of that part of the

transformer is transferred to C808 and R807. Because of factors such as lead inductance of each element, resistance of the

capacitor circuit, frequency characteristics of the parasitics, we can only do an approximate analysis. As the diode D802

has a short recovery time, the reverse current can be disregarded.

R803

VIN

VG

R804

R806

R824

C824

R808

R809

Vgs = Gate–Source voltage

Vgs(th) = Gate–Soure threshold voltage

< Drawing 1: motive/driving stage of circuit >

R811

C808

D802

R807

C809

L804

< Drawing 2: snubber circuit >