Схемы, платы, компоненты

DK3704A – High‑Frequency, High‑Efficiency Synchronous Rectifier Controller

Version: 1.3 – July 2025 – http://www.dkpower.cn

Product Overview

The DK3704A is a synchronous rectifier controller chip. When paired with an external power MOSFET, it can replace Schottky diodes to provide rectification, greatly improving system efficiency. It is suitable for QR flyback, active clamped flyback (ACF), asymmetric half‑bridge (AHB) and other topologies.

Efficiency‑Enhancing Features

Fast comparator with short propagation delay reduces switching loss.

Higher gate‑drive voltage lowers MOSFET conduction loss.

Built‑in standby mode further reduces standby power consumption.

Output‑voltage‑powered operation minimizes supply loss.

Adaptive Slope‑Based Anti‑Mis‑fire (Anti‑Burst) Algorithm

The chip uses an internal adaptive slope protection method that requires no external parameters, distinguishing normal on‑time from resonance to prevent erroneous turn‑on in discontinuous‑mode operation.

Package

SOT23‑6 (6‑pin Small Outline Transistor).

Main Features

Suitable for flyback QR, active clamped flyback (ACF), and asymmetric half‑bridge (AHB) applications.

Low supply loss: power is taken from the system output.

Adaptive anti‑mis‑fire algorithm.

Switching frequency: 500 kHz.

Standby mode with ultra‑low standby current.

Minimal external components.

Typical Applications

USB chargers

Power adapters

LED drivers, etc.

Pin‑out & Function Description

| Pin No. | Pin Name | Description |

| 1 | NC | Float (no connection) |

| 2 | GND | System ground and the source terminal of the power MOSFET |

| 3 | GT | Gate of the power MOSFET |

| 4 | K | Synchronous detection pin, connected to the drain of the power MOSFET |

| 5 | NC | Float (no connection) |

| 6 | VCC | Supply pin powered from the system output voltage |

Version: 1.3 – July 2025 – http://www.dkpower.cn

Circuit Schematic (see figures in the datasheet)

1. Chip Startup and Shutdown

The chip requires an external power supply; exceeding Vcc_max will damage the device.

When the output voltage falls below the start‑up voltage VCC_ON, the power MOSFET turns off and current flows via the body diode.

When VCC exceeds VCC_ON, the chip exits start‑up mode and can normally control the MOSFET on/off.

If VCC drops below the reset voltage VCC_OFF, the chip restarts.

2. GT Voltage Control

The external MOSFET gate voltage is supplied by VCC.

Internally, the chip determines the gate‑drive voltage based on VCC:

VCC < Vgt_max → GT = VCC

VCC ≥ Vgt_max → GT = Vgt_max

3. NMOS Control

The chip continuously monitors the K pin voltage.

When K voltage falls below the turn‑on voltage VON, the MOSFET is turned on.

When K voltage rises above the turn‑off voltage VOFF, the MOSFET is turned off.

After the MOSFET is on, the voltage at K is determined by the MOSFET’s resistance and the current through it.

When K voltage rises to the pre‑shutdown voltage VPRE, an internal pre‑shutdown function activates: a certain current discharges part of the charge, lowering the gate voltage and ensuring a fast turn‑off.

4. Anti‑Mis‑fire (Anti‑Burst) Protection

In discontinuous‑mode (DCM) operation, if the resonant valley voltage reaches VON, the synchronous chip may turn on erroneously. Reverse current through the MOSFET creates a high spike at K, reducing efficiency and possibly damaging the chip.

The chip detects the slope of the falling voltage at K during normal turn‑on versus resonance and uses this to prevent mis‑fire in DCM.

The slope‑based reference is internally adaptive, requiring no external tuning, so mis‑fire is prevented while normal on‑time cycles remain unaffected.

5. Standby Mode

The chip determines standby by measuring the number of synchronous discharge cycles within a time window Tstd.

Standby condition: fewer than 8 discharge cycles detected → the chip enters standby, the power MOSFET stays off, and static power consumption is extremely low.

Exit standby: more than 32 discharge cycles detected → the chip resumes normal operation.