| SLA Status | Assignee | Support Status |
|---|---|---|
| ⚠ 648 hours | Awaiting Company Response | |
LT8309
Production
The LT8309 is a secondary-side synchronous rectifier driver that replaces the output rectifier diode in a flyback topology. By replacing the diode with...
Datasheet
LT8309 on Analog.com
Hello Folks,
The issue is premature turn off. As the the drain voltage falls below the set threshold (-10mV programmed by 3K resistor) the gate (after a small delay) turns on.
The gate then turns off 400-500ns later caused by positive swings of the drain voltage.
Ideally, if the chip had a 200-300ns longer blanking period all would be good. ( see images).
I had experimented with 3k , 2.6K, 2.15K to no avail.
In any case what do you recommend is to be done to avoid the false triggering.
the ideas are:
- 470pF+20ohm snubber across the MOSFET.
- Exchange the MOSFET for for a 15-20mOhm one. (Current MOSFET is Ron=7.5mohm)
Images purple is gate yellow is drain
for those interested in deeping into SR issues...(using the LT8316 and LT8309 in a 150W nominal - 180W peak design )
Just received the MOSFETs form INFINEON.
As shown in the simulation models above (SPICE L0 for the 13mOhm and 20mOhm MOSFETs) the new MOSFETs address the issue. In essence, I have confirmed experientially the Q2drain is now pulled far enough below ground to stabilize the SR system. (see waveforms)
The system produces (as has been modeled) 24V out at a peak efficiency of 93.1% at a nominal power of 150W. The efficiency at 170W output is still 92%. The converter has been under extended stress tests at 170W output with no issues. (all thermal imaging shows <70C for all components)
Also note: add ons:
1. The SR stage in its final form has a 150V zener across the MOSFET to clamp the voltage and protect the drain pin on the LT8309.
2. We will be adding 4.7pF to the built in 15pF capacitance of the drain pin to further lengthen the on delay and to filter additional unwanted ringing. this improves sub 50W performance when using 13mOhm and 20mOhm MOSFETs.
3. the drain pin will be protected by a 130V zener to further reduce the risk of damage to the pin.
We will issue a detailed 0-180W performance report once complete.
Thank you for reading
Kindest regards
See below: waveform shows this QR BCM converter (w 13mOhm secondary MOSFETs ) producing
170W continuous @ 24V with no issues. (yellow is Q2 drain and purple is Q2 gate) scope is 200MHz in 14 bit mode
130W continuous with no issues. (yellow is Q2 drain purple is Q2 gate).
for those interested in deeping into SR issues...(using the LT8316 and LT8309 in a 150W nominal - 180W peak design )
Just received the MOSFETs form INFINEON.
As shown in the simulation models above (SPICE L0 for the 13mOhm and 20mOhm MOSFETs) the new MOSFETs address the issue. In essence, I have confirmed experientially the Q2drain is now pulled far enough below ground to stabilize the SR system. (see waveforms)
The system produces (as has been modeled) 24V out at a peak efficiency of 93.1% at a nominal power of 150W. The efficiency at 170W output is still 92%. The converter has been under extended stress tests at 170W output with no issues. (all thermal imaging shows <70C for all components)
Also note: add ons:
1. The SR stage in its final form has a 150V zener across the MOSFET to clamp the voltage and protect the drain pin on the LT8309.
2. We will be adding 4.7pF to the built in 15pF capacitance of the drain pin to further lengthen the on delay and to filter additional unwanted ringing. this improves sub 50W performance when using 13mOhm and 20mOhm MOSFETs.
3. the drain pin will be protected by a 130V zener to further reduce the risk of damage to the pin.
We will issue a detailed 0-180W performance report once complete.
Thank you for reading
Kindest regards
See below: waveform shows this QR BCM converter (w 13mOhm secondary MOSFETs ) producing
170W continuous @ 24V with no issues. (yellow is Q2 drain and purple is Q2 gate) scope is 200MHz in 14 bit mode
130W continuous with no issues. (yellow is Q2 drain purple is Q2 gate).
