The UCC256404B maximum XCD test current is lower (1.3mA) compared to the maximum XCD test currents of the 404 and 404A (1.7mA). This difference helps to reduce stand-by power loss when using the UCC256404B during very-light-load to no-load conditions.

Figure 2-1 shows the startup procedure for UCC256402 and UCC256404 devices. Here we can observe that RVCC will be enabled only when VCC reaches 26 V.

Figure 2-1 HV Startup for 402 and 404 Devices

As mentioned in Table 1-1, 403 devices does not have an HV startup. Auxiliary supply is needed to startup these devices. Figure 2-2 shows the startup procedure for UCC256403 devices with auxiliary supply.

Figure 2-2 HV Startup with External Bias for 403 Devices

• Figure 2-3 shows the internal architecture of the HV, VCC, X-cap discharge block.
• The switch S1 shown in Figure 2-3 controls high voltage startup for the device. Once the VCC pin exceeds the VccStartSelf (402 and 404 devices), this switch will be turned off which means HV startup is finished.

Figure 2-3 HV, VCC, X-cap Discharge Internal Block Architecture

The typical recommended resistor value for R_HV shown in Figure 2-4 is 5k Ohm.

Figure 2-4 HV Startup Block Diagram

Figure 2-5 Voltage at HV Pin for Zero Crossing Detection

• IEC60950 and IEC60065: 1s after AC disconnect, the remaining voltage on x-capacitor should be less than 37% of the initial voltage.
• IEC62368: 2s after AC disconnect, the remaining voltage on x-capacitor should be less than 60 V.

• HV pin is connected to Line and Neutral through two rectifier diodes as shown in Figure 2-4; This pin’s voltage is used to detect if AC is present.
• Under ideal conditions when AC is connected, the HV pin should detect the zero crossing where the voltage between Line and Neutral reaches 0 V twice per cycle. However, due to the parasitic capacitance of the diode bridge, the voltage at HV will not reach zero. To overcome this, current is sinked from HV to GND.

Figure 2-6 X-cap discharge internal block diagram

• The switch S2 highlighted in Figure 2-6 controls the Ixcap test current to determine if AC is still connected. The logic is looking for the HV pin to be pulled below 9V. Note this test current goes to gnd.

Figure 2-7 Xcap Discharge

• Test current I1 is applied to check if HV pin voltage is <9 V threshold.
• When HV pin voltage falls below 9 V, test current is turned off and controller waits 700 ms before applying I1 again.
• I1 test current is on for maximum of 12 ms. If HV pin does not satisfy <9 V requirement, test current is increased to I2, then I3 and finally, I4.
• I4 test current maximum on time is 48 ms.
• If zero crossing is not detected during the test current period, a discharge current of 11.5 mA (typical value) is applied for 350 ms from HV to GND as shown in Figure 2-7.

Figure 2-8 Test Condition: AC Plugged in, BLK UVLO; Ch1 – LO, Ch2 – VCC, Ch3 – RVCC, Ch4 – HV

Figure 2-9 Zoomed In: Test Condition: AC Plugged in, BLK UVLO; Ch1 – LO, Ch2 – VCC, Ch3 – RVCC, Ch4 – HV

In Figure 2-8 and Figure 2-9, we can observe that whenever test current being injected (for every 700 ms) HV pin voltage is being pulled down. Also, when VCC is reduced to VCCRestartJFET threshold, VCC charging is initiated.

Figure 2-10 Waveform Capture of AC Input Removal and X-cap Discharge

As UCC25640x is a primary side LLC controller, the output voltage/current is regulated by a voltage/current regulator circuit located on the secondary side of the isolation barrier. The demand signal from the secondary side regulator circuit is transferred across the isolation barrier via an opto-coupler (U1) as shown in Figure 2-11.

Figure 2-11 Feedback Chain Block Diagram

Figure 2-12 FBreplica Generation

• FB pin is supplied with an IFB current source
• The optocoupler collector voltage maintained at approximate constant voltage (~5.6 V) when FB sourcing current is less than IFB. Due to this, no extra pole introduced due to the optocoupler parasitic capacitor
• The FB clamp circuit is to provide extra 82uA current to the FB pin when needed to prevent FB voltage drop and to maintain good transient performance
• The Optocoupler current is reflected as the FBreplica in the IC for control purpose
• Voltage at the RFB is given by FBreplica = (IFB-Ioptocoupler) *RFB where RFB typical value is 100 kohm and IFB typical values are 82 uA (for 402, 404 devices) and 164 uA (403, 403A devices).
• If the current through optocoupler is zero during the heavy output load, FB pin voltage will be clamped to internal whereas FBreplica is clamped to 6 V.