TNY280 Under Voltage
I have a design using a TNY280 and I am wondering about the EN/UV pin. The supply is the bias supply for an OLS and the EN/UV pin is connected to the output of the Boost converter 400V bus with a 3.3Meg ohm resistor. What I noticed is if the input line is reduced to below the UV for the boost converter the TNY will continue to run. The boost converter drops below 30V and the TNY280 still runs. I would like to turn the bias supply off if the boost voltage drop to less than 70V or so.
I have also noticed that the TNY280 starts switching when the Boost voltages is as low as 11V and I have increased R601 to 6.6Meg ohms. Under that condition the current into EN/UV is well under 25uA so how does the TNY280 start and run with such low current into EN/UV?
Please ignore the unattached components in the schematic.
Comments
You are correct. Loss of regulation is defined as the EN/UV pin not being pulled low for >64 ms ie the opto is not being driven because the output voltage is below the regulation point.
As a stand alone converter this works well - the logic being as long as the output is in regulation then the fact the input voltage is low is acceptable (note TinySwitch will not be damaged by operation at low input voltages). This also maximizes hold-up time, allowing the output the be kept in regulation for as long as possible as the bulk capacitor discharges.
For references here's the section in the datasheet:
Line Undervoltage Sense Circuit
The DC line voltage can be monitored by connecting an external resistor from the DC line to the EN/UV pin. During power up or when the switching of the power MOSFET is disabled in auto-restart, the current into the EN/UV pin must exceed 25 μA to initiate switching of the power MOSFET. During power up, this is accomplished by holding the BYPASS/MULTIFUNCTION pin to 4.9 V while the line undervoltage condition exists. The BYPASS/MULTI-FUNCTION pin then rises from
4.9 V to 5.85 V when the line undervoltage condition goes away. When the switching of the power MOSFET is disabled in auto-restart mode and a line undervoltage condition exists, the auto-restart counter is stopped. This stretches the disable time beyond its normal 2.5 seconds until the line undervoltage condition ends.
The line undervoltage circuit also detects when there is no external resistor connected to the EN/UV pin (less than ~2 μA into the pin). In this case the line undervoltage function is disabled.
The 11 V still puzzles me. Check to see if the 11 V condition occur after a cold start (everything discharged and BP pin voltage <3V). This ensures IC is reset for the experiment. Increase input voltage slowly. Let me know at what voltage (with 3.3 M ohm resistor connected from DCbus to EN/UV pin) output goes into regulation (TNY280 drain starts switching). Note you may see some small wiggles (~5 V) on the drain pin due to the internal supply (to BP pin) - when out UV+ threshold is reached the drain will be pulled all the way down to zero.
Cheers
PI-Chekov
"the easiest way to do this is a bipolar connected to the EN/UV pin with the base of the bipolar connected to resistor divider connected to the dc bus"
Could you provides more detail? I dont quite understand this.
Hello vantubk,
Thank you very much for using Power Integrations products and sorry for the late reply.
the UV pin is used to detect during start up and auto-restart. The DC line voltage can be monitored by connecting an external resistor from the DC line to the EN/UV pin. During power up or when the switching of the power MOSFET is disabled in auto-restart, the current into the EN/UV pin must exceed 25 mA to initiate switching of the power MOSFET. During power up, this is accomplished by holding the BYPASS/MULTIFUNCTION pin to 4.9 V while the line undervoltage condition exists. The BYPASS/MULTI-FUNCTION pin then rises from 4.9 V to 5.85 V when the line undervoltage condition goes away. When the switching of the power MOSFET is disabled in auto-restart mode and a line undervoltage condition exists, the auto-restart counter is stopped. This stretches the disable time beyond its normal 2.5 seconds until the line undervoltage condition ends. The line undervoltage circuit also detects when there is no external resistor connected to the EN/UV pin (less than ~2 mA into the pin). In this case the line undervoltage function is disabled.
if you want to have UV function during normal operation. The "easiest way" here he mentioned is to use a resistor divider to sense the line voltage and use a bipolar to trigger the protection. with the bipolar base connected to the resistor divdider and the emitter connected to BP and the collector connected to EN/UV with a resistor in seires to limit the current.
Here it use the auto-restart function of the EN/UV pin not the UV function. An internal counter clocked by the oscillator is reset every time the EN/UV pin is pulled low. If the EN/UV pin is not pulled low for 64 ms, the power MOSFET switching is normally disabled for 2.5 seconds (except in the case of line undervoltage condition, in which case it is disabled until the condition is removed).
When the line voltage is under voltage the BJT will be turned on and more current will be injected into EN/UV pin to trigger the auto-restart. But it may also affect your other perofrmance, output ripple or efficiency. you should check after you add this circuit.
Best Regards
Hello there,
The best way to describe the UV feature (via the EN/UV pin) is that it is a UV+ threshold. It defines the input voltage where the IC will start to operate. Once operating the IC continues to switch until the output looses regulation - there is no defined voltage threshold (ie no UV-) via where the IC will stop operating. The UV+ threshold is reached when 25 uA current is provided into the EN/UV pin. For a 3.3 M ohm this would be about 81 V.
If a defined UV- threshold is required then this has to be added externally, the easiest way to do this is a bipolar connected to the EN/UV pin with the base of the bipolar connected to resistor divider connected to the dc bus.
What does seem odd is the statement that the TNY280 starts when the boost voltage is 11 V - that should not happen. What does the layout look like - any high voltage traces close to the EN/UV pin? We have seen issues with noise and leakage (through flux) when the DC bus, drain or any high voltage trace is close to a node connected to he EN/UV pin.
Cheers
PI-Chekov