Hello,
I’am using a MP8007 in flyback mode for a POE application.
The circuit is designed to get 12V/1A.
I noticed my design works only if I consomme at least 30mA.
Othewise, the negotiation with PSE seems to failed and and the output voltage is not OK.
Is it normal to need a minimum current consumption to get a normal behviour of the MP8007 circuit?
Best regards,
Julien
Hello Julien,
When operating in flyback mode, the converter needs a dummy load. This is because the output voltage rises above normal operation voltage due to the minimum switching frequency limitation. If you are following figure 14 in the datasheet:
A minimum load of 10mA is recommended.
Please read section labeled “Converter Dummy Load” for more information.
Thank you,
Vinh Tran
Field Applications Engineer
I think deep in the spec, the PSE is allowed to disconnect if you don’t draw some minimum load.
Thanks @vinh.tran and @jshannon for your answers.
I also noticed that depends on the PSE. 10mA is OK for one and 30mA for another one.
Anyway, it is coherent with the section “Convertr Dummy Load”.
On the same project, I have another board with a MP8009 used in Flyback mode (5V/5A).
For this one, I have to connect a load of 200mA to get the voltage.
That’s seems to me very large. Moreover, there a no mentions of the "dummy load’ in the MP8009 datasheet.
Could you help me?
Best regards,
Julien
Hello Julien,
This is may be due to the light-load control.
In SSR mode, if the COMP voltage drops too low, then it reaches the power-save mode.
In PSR mode, if the sensed peak current is below 36mV, then instead of reducing transformer current it reduces the frequency. If the frequency is below 30kHz without a load, Vout rises and triggers OVP.
If you would like to read more, then please read the section titled “Light-Load Control” on page 24-25 of the MP8009 datasheet.
Thank you,
Vinh Tran
Field Applications Engineer
Hello Vinh,
In my case, I am in PSR mode (without opto-coupler).
Can I change any parameters to be less sensitive to light loads?
Maybe, change the Rsense resisor to increase the seanse peak current?
Thank you,
Julien
I think the question from the OP - is why does the PSE cut the MP8007 off without at least a 10 ma or 30 ma load on the 12v output.
This is expected behaviour - the PoE Switch standard shuts the port down if there isn’t a minimum load of about 300 mW. The newer versions of PoE like 802.3bt allow a lower minimum, but your customers probably have 802.3af and 802.3at PSE.
I understand and I am OK fir the MP8007 with 10ma or 30ma load, depends of the PSE.
But for the MP8009, I find that 200mA for 5V is very high (1W)…
the requirement from the PSE is milliamps at 48v - that means about 300 mW - so that is 25 mA at 12v or 60 mA at 5v. Why do you think you need 200 mA at 5v?
I think that is what he is observing.
Yes, It is what I am observing.
So that is odd, is it maybe a timing issue? The part is bursting so sometimes drawing several watts and sometimes drawing nothing. Gets a disconnect in the nothing phase.
I would be tempted to put a 300mW resistive load on the primary side after the PD switch and see what happens.
thanks my issue has been fixed.
thanks my issue has been fixed.