UCC9950EVM-631 Feature Testing
6.7
On/Off Control of PFC Stage and PFC/LLC Stages
In Aux Bias Mode only. When the EVM is operating in Aux Bias Mode the switches S1 and S2 may be
used to turn the LLC stage off and on or turn both the LLC and PFC stages off and on. Add a ‘scope
probe to the MD_SEL/PS_ON pin at TP15. Get the EVM operating normally in Aux Bias Mode as
described earlier, TP15 should be at 0 V and then go to VCC when S2 is put in the ‘on’ position, see
. Then turn both stages off by flipping S2 back into the ‘off’ position, see
. TP15 should go
from VCC to 0 V. Then, turn the PFC stage alone on by flipping S1 into the ‘on’ position, see
TP15 should go to approximately VCC/2. Observe that the LLC stage has stopped (V
OUT
= 0 V) and the
PFC stage is running (V2 monitoring the bulk capacitor). Flip S2 into the ‘on’ position again (S1 may be
left in the ‘on’ position), TP 15 should go to VCC and the LLC stage will start. This feature allows the user
additional flexibility in system design. Be careful when operating these switches because hazardous
voltages exist on the PCB.
6.8
Three Level LLC Over-Current Protection for Loads with High-Peak Power
Requirements
The UCC29950 includes a three level over current protection feature on the LLC stage which allows for
short term overloads beyond the normal current limit point (28A).
Increase the load on the output slowly, the over current protection will operate at about 28 A. The OCP
feature stops the LLC operating and then tries to restart it after about 1 second. The 28 A overload
protection is triggered by the OCP1 level and will trip after a nominal 52 ms delay.
Set the electronic load to apply an overload load transient (25 A to 33 A to 25 A) for a short period (20
ms). This will not trip the over current protection. Increase the time period incrementally until the OCP1
over current protection is activated.
The second OCP level (OCP2) operates at about 42 A after 10 ms. This should be observed in the same
way as the OCP1. Set the load to apply an overload load transient (25 A to 50 A to 25 A) for a short
period 5 ms. This will not trip the over current protection. Increase the time period incrementally until the
over current protection is activated as before.
The third OCP level (OCP3) provides protection against output short circuits.
6.9
Short Circuit Protection
The UCC29950 also includes short circuit protection which operates immediately the signal at the LLC_CS
pin exceeds 900 mV (OCP3). Test this by applying a short circuit to the EVM output terminals, the LLC
stage stops immediately. The best way to observe this is by monitoring one of the LLC gate drive signals
at TP12 and the LLC_CS signal at TP16. Switching action will stop immediately if the signal at TP16
exceeds 900 mV.
6.10 Current Sense Inputs for PFC Overload Protection
The PFC stage current sense input is used as an input to the control loop in the UCC29950. The
UCC29950 uses the PFC_CS signal to shape the input current during the line cycle. It also has a current
and power limiting function as explained in the ‘PFC Stage Current Sensing‘ and ‘Input Power Limit’
sections of the data sheet. The operating point for both these features is set at a level which is higher than
the point at which the LLC OCP protections operate so they are not normally triggered.
16
Using the UCC29950EVM-631 300-W PFC/LLC Off-Line PSU Module
SLUUB69A – March 2015 – Revised March 2015
Copyright © 2015, Texas Instruments Incorporated