8.3.1 High-Voltage Current Source Start-Up Operation

The controller includes a switched, high-voltage, current source on the HV pin to allow fast start-up, and eliminates the static power dissipation in a conventional resistive start-up approach. This feature reduces standby power consumption.

The HV pin has three major functions:

• Supply the device start-up current
• Supply the device bias power during latched fault mode
• AC sense input for X-capacitor discharge detect (UCC28630 and UCC28633 only)

The UCC28630 and UCC28633 input supply to the HV start-up pin must be connected to the AC side of the bridge rectifier as shown in Figure 15, in order to support X-capacitor discharge. More details are given in Active X-Capacitor Discharge (UCC28630 and UCC28633 only), below. Connection to the AC side of the bridge also allows faster detection of AC mains removal under latched fault conditions, allowing prompt reset of latched faults for fast restart.

Figure 15. HV Pin Connection: (a) AC-side, (b) DC-side (UCC28631, UCC28632 and UCC28634 only)

In the UCC28631, UCC28632 and UCC28634, the HV pin can connect to either the AC or DC side of the bridge. The addition of the 200-kΩ external HV resistance (required for X-capacitor discharge sensing) limits the available charging current for the external bias supply input capacitor. However, for typical values of between 22 µF and 33 µF of input capacitance, start-up bias times of less than 1.5 s are achievable at 90 V_AC. Start-up time can be estimated using Equation 1.

Equation 1.

where

• for AC connection and V_IN(avg) = V_RMS x √2 for DC connection

For 90 V_AC, if C_VDD = 22 µF and worst case V_{DD(start_max)} = 16.5 V, then t_START is 1.002 s.

Figure 16 illustrates the start-up behavior of the controller. The HV current source has built-in short-circuit protection that limits the initial charge current out of the bias voltage pin until the bias voltage reaches V_DD(sc). This limits the power dissipated in the HV current source in the event of a short circuit on the VDD pin. Thereafter, the HV current source switches to full available current. The controller remains in a low-power, start-up mode until the bias voltage reaches V_DD(start), after which the HV current source is turned off and the controller initiates a start-up sequence.

The bias voltage decays during the start-up sequence at a rate dependent on the size of the energy storage capacitor connected to the VDD pin. The VDD storage capacitor must be sized appropriately to ensure adequate energy storage to supply both the controller bias power and MOSFET drive power during start-up, until the VDD rail can be supplied through the transformer bias winding. If the bias voltage falls below V_DD(stop) (due to bias winding fault or an inadequate VDD storage capacitance), the controller stops switching, and transitions into low-power mode for a time delay of t_RESET(long), or until the bias voltage falls to the V_DD(reset) level, whichever is shorter. See VDD Capacitor Selection for required VDD capacitor sizing. Once the time delay elapses, the bias voltage rapidly discharges to the V_DD(reset) level, followed by turn-on of the internal HV current source, and a normal restart attempt follows.

Figure 16. Normal Start-Up Sequence,
(assuming V_AC > UV start threshold)