Using Bias Controllers in USB Type-C ACDC Applications

    The USB Type-C standard allows for an adjustable output voltage ranging from 5V to 20V and load currents up to 3A using a standard cable. With power levels reaching up to 60W, the flyback topology remains a good choice. However, providing bias power to the primary-side controller can present some challenges.

    An auxiliary winding on the flyback transformer typically powers the controller on the primary side of the isolation boundary. This winding generates a voltage proportional to the output voltage. With a 4-to-1 output voltage range, the bias voltage also varies by a factor of 4-to-1. In practice, the actual range is wider to account for peak charging of the bias capacitor due to ringing.

    We must set the turns ratio from the output winding to the auxiliary winding so that the bias voltage is high enough to effectively drive the primary MOSFET when the output is only 5V. For a drive supply of approximately 12V, a ratio of 1 to 2.5 (output to auxiliary) might be a good choice. However, this means that when the output voltage is 20V, the auxiliary voltage will exceed 50V. Clearly, we must take steps to protect the controller from overvoltage damage.

    Adding a simple clamp circuit (as shown in Figure 1) provides a good solution. The transistor (Q1) must have a fairly high gain to ensure that the bias voltage does not drop when the output is at 5V. The clamp voltage is set by the Zener value (D10). The base resistor (R27) must be set low enough to provide the necessary base current at 5V output, but not too low. A base resistor value that is too low will result in unnecessary losses.

    Figure 1: A clamp circuit is needed to limit the voltage on the bias rail

    At first glance, this clamping circuit appears to significantly increase standby (no-load) power loss. However, in the no-load condition with the USB Type-C cable unplugged, the output voltage defaults to 5V. When the output is 5V, the clamping circuit adds almost no additional power loss. Even with this additional clamping, keeping standby power loss below 50mW is fairly straightforward.

    The UCC28740 isolated flyback power supply controller uses an optocoupler to provide constant voltage (CV) to improve transient response to large load steps. Constant current (CC) regulation is achieved through primary-side regulation (PSR). The device processes information from the optocoupler feedback and the auxiliary flyback winding to achieve precise, high-performance control of the output voltage and current.

    An internal 700-V startup switch, dynamically controlled operating states, and customized modulation profiles enable ultra-low standby power without sacrificing startup time or output transient response.

    The control algorithm in the UCC28740 allows for operating efficiencies that meet or exceed applicable standards. The driver output interfaces with the MOSFET power switch. Discontinuous conduction mode (DCM) with valley switching reduces switching losses. Modulation of the switching frequency and primary current peak amplitude (FM and AM) maintains high conversion efficiency across the entire load and line range.

    The controller operates at a maximum switching frequency of 100 kHz and consistently controls the transformer's peak primary current. Protection features limit stress on primary and secondary components. A minimum switching frequency of 170 Hz helps achieve a no-load power draw of less than 10 mW.

    ● Features:

    ■ No-load power less than 10 mW

    ■ CV uses optical coupling feedback, CC uses primary side regulation (PSR)

    ■ ±1% voltage regulation and ±5% current regulation on line and load

    ■ 700V starting switch

    ■ 100 kHz maximum switching frequency enables high power density charger designs

    ■ Resonant valley switching operation, the highest overall efficiency

    ■ Frequency jittering to ease EMI compliance

    ■ MOSFET clamp gate drive output

    ■ Overvoltage, low voltage line and overcurrent protection functions

    ■ SOIC-7 package

    ■ Create custom designs using the UCC28740 and WEBENCH® Power Designer