HVLED805

Application information

Doc ID 18077 Rev 1

15/29

5.3

Secondary side demagnetization detection and triggering

block

The demagnetization detection (DMG) and Triggering blocks switch on the power MOSFET

if a negative-going edge falling below 50 mV is applied to the DMG pin. To do so, the

triggering block must be previously armed by a positive-going edge exceeding 100 mV.

This feature is used to detect transformer demagnetization for QR operation, where the

signal for the DMG input is obtained from the transformer’s auxiliary winding used also to

supply the IC.

The triggering block is blanked after MOSFET’s turn-off to prevent any negative-going edge

that follows leakage inductance demagnetization from triggering the DMG circuit

erroneously.

The voltage on the pin is both top and bottom limited by a double clamp, as illustrated in the

internal diagram of the DMG block of Figure 12. The upper clamp is typically located at 3.3

V, while the lower clamp is located at -60mV. The interface between the pin and the auxiliary

winding will be a resistor divider. Its resistance ratio as well as the individual resistance

values will be properly chosen (see “Section 5.5: Constant current operation on page 18”

and “Section 5.6: Voltage feedforward block on page 20”.

in all the Vin range conditions. No capacitor is allowed between DMG pin and the auxiliary

transformer.

The switching frequency is top-limited below 166 kHz, as the converter’s operating

frequency tends to increase excessively at light load and high input voltage.

A Starter block is also used to start-up the system, that is, to turn on the MOSFET during

converter power-up, when no or a too small signal is available on the DMG pin.

The starter frequency is 2 kHz if COMP pin is below burst mode threshold, i.e. 1 V, while it

becomes 8 kHz if this voltage exceed this value.

Figure 12.

DMG block, triggering block

60mV

DMG

CLAMP

BLAN KIN G

TIME

TUR N-ON

LOGIC

STAR TER

S

R

Q

LEB

Aux

Rfb

Rdmg

To Driver

From CC/C V Block

From OCP

DMG

110mV