STMicroelectronics HVLED805 Evaluation Board EVALHVLED805 EVALHVLED805 Data Sheet
Product codes
EVALHVLED805
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.
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.
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.
that follows leakage inductance demagnetization from triggering the DMG circuit
erroneously.
This blanking time is dependent on the voltage on COMP pin: it is T
BLANK
= 30 µs for V
COMP
= 0.9 V, and decreases almost linearly down to T
BLANK
= 6 µs for V
COMP
= 1.3 V
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
internal diagram of the DMG block of
. 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 “
winding will be a resistor divider. Its resistance ratio as well as the individual resistance
values will be properly chosen (see “
”
and “
”.
Please note that the maximum I
DMG
sunk/sourced current has to not exceed ±2 mA (AMR)
in all the Vin range conditions. No capacitor is allowed between DMG pin and the auxiliary
transformer.
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.
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.
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.
becomes 8 kHz if this voltage exceed this value.
Figure 12.
DMG block, triggering block
60mV
D MG
CLAMP
BLAN KIN G
TIME
TUR N-ON
LOGIC
STAR TER
S
R
Q
LEB
+
-
Aux
Rfb
R dmg
To Driver
From CC/C V Block
From OCP
DMG
110mV