Introduction

The demand for smaller, lighter, and extra environment friendly AC/DC USB energy supply (PD) chargers is at all times a problem for power-supply design engineers. Beneath 100 W, the quasi-resonant flyback remains to be the dominating topology, and gallium nitride (GaN) expertise can push the facility density and effectivity additional.

Nevertheless, offering bias energy for the first controller requires an auxiliary winding on the transformer in addition to rectifying and filtering circuitry. To make issues worse, the USB PD charger output voltage has a variety. For instance, the USB PD commonplace energy vary covers output voltages from 5 V to twenty V, and the most recent USB PD prolonged energy vary permits the output voltage to go as excessive as 48 V. For the reason that auxiliary voltage is proportional to the output voltage, the bias voltage vary on the first controller will improve, requiring additional circuitry and degrading effectivity. On this energy tip, I’ll introduce a self-biased flyback converter resolution to handle these design challenges.

Coping with huge bias voltages

Determine 1, Determine 2, Determine 3, and Determine 4 present 4 other ways to take care of the huge bias voltage vary in USB PD charger functions. Standard strategies embrace utilizing a linear regulator, a tapped auxiliary winding, and even including an additional DC/DC switching converter to manage the bias voltage. All of those strategies will improve part depend, add price, or improve energy losses. Alternatively, self-biasing completely removes exterior elements and will increase effectivity.

Determine 1 Bias circuits for functions with huge output voltage ranges utilizing a discrete linear regulator. Supply: Texas Devices

Determine 2 Bias circuits for functions with huge output voltage ranges utilizing a tapped auxiliary winding. Supply: Texas Devices

Determine 3 Bias circuits for functions with huge output voltage ranges utilizing increase converter. Supply: Texas Devices

Determine 4 Bias circuits for functions with huge output voltage ranges utilizing a self-biased VCC. Supply: Texas Devices

VCC self-biasing

The flyback controller can at all times get bias energy straight from the rectified AC enter voltage, however this ends in extreme energy losses. The important thing to self-biasing is to reap power from the facility stage, which may come from two sources. One is the switch-node capacitor saved power; the opposite is power saved within the primary-side winding of the transformer. As proven in Determine 5, an built-in self-biasing circuit can ideally do each, primarily based on the enter and output situations.

Determine 5 The self-bias circuit harvests power from the switch-node capacitance or magnetizing inductance. Supply: Texas Devices

Determine 6 reveals the power harvesting from the switch-node capacitor. This may save effectivity because it recycles the power storage in switching node capacitor in each switching cycle. In circumstances reminiscent of AC low-line enter when the mirrored output voltage is an identical to the enter voltage, pure zero voltage switching will happen, and there’s no power within the switch-node capacitor, inductor power harvesting will take impact, the place a small portion of the first switching present is directed to the VCC cap via an inside path.

Determine 6 VCC self-bias operation: (a) capacitor power harvesting on the switching node and (b) inductor power harvesting via the first present. Supply: Texas Devices

Attaining auxless sensing

Many flyback controllers use the auxiliary winding to sense the enter and output voltages and detect situations reminiscent of output overvoltage or enter undervoltage. With self-biased flyback converters, it’s doable to make use of the switching-node voltage for enter and output voltage sensing. As proven in Determine 7, the sensed voltage is the sum of the enter and mirrored output voltage. For the reason that common voltage throughout the first winding is zero, the typical of the switch-node voltage is the same as the enter voltage.

For output voltage sensing, it might pattern the mirrored output voltage, and the controller must be knowledgeable of the precise turns ratio of the transformer with the usage of a resistor-programmable pin [the TR pin in the Texas Instruments (TI) UCG28826].

Determine 7 Auxless voltage sensing the place the sensed voltage is the sum of the enter and mirrored output voltage. Supply: Texas Devices

As soon as correctly configured, self-biased gadgets such because the UCG28826 can precisely present varied protections like overpower and overvoltage safety. Determine 8 reveals the UCG28826 in a USB PD utility.

Determine 8 A self-biased USB PD design utilizing the UCG28826 that may precisely present varied protections like overpower and overvoltage safety. Supply: Texas Devices

Determine 9 reveals the overvoltage safety waveforms after deliberately disconnecting the suggestions pin which is a single fault situation. The controller senses the output voltage and triggers overvoltage safety accordingly when the output ramps as much as round 24.4 V for a nominal 20 V output.

Determine 9 Auxless sensing instance for overvoltage safety. Channel 1 (CH1) is Vout and channel 2 (CH2) is Vsw. Supply: Texas Devices

Prototype and check end result

Determine 10 reveals the TI common AC-input 65W twin USB type-C port USB PD charger reference design with an built-in GaN energy change. Because of the simplified self-bias characteristic and built-in GaN change within the UCG28826, the reference design achieves an influence density of two.3 W/cm³ and 93.2% effectivity for the AC/DC stage. The auxless design additionally simplifies transformer manufacturing and reduces prices. Desk 1 summarizes the design parameters of 65 W design for reference. 

Determine 10 A common AC-input 65-W reference design board. Supply: Texas Devices

Desk 1 Common AC-input 65W reference design parameters.

Simplified USB PD charger

A high-level integration with a controller and GaN change can simplify USB PD charger design, however the bias circuitry for the controller and related auxiliary winding on the transformer are nonetheless there, degrading effectivity and affecting measurement and value. An built-in self-biasing circuit can eradicate that portion of the circuit and improve the facility density for energy provides with wide-range outputs. Moreover, it’s nonetheless doable to attain correct enter and output voltage sensing within the absence of an auxiliary winding on the transformer.

Max Wang is a methods engineer and Member, Group Technical Employees at Texas Devices. He has over 18 years of expertise within the energy semiconductor and power-supply industries in computing, industrial, and private electronics markets; specializing in remoted AC/DC and DC/DC functions. His design and analysis pursuits embrace high-efficiency and high-power-density energy conversion, soft-switching converters, and GaN implementation in AC/DC converters. Max obtained a grasp’s diploma in electrical engineering from Zhejiang College in 2006. He has labored at Delta, Energy Integrations, Infineon and Texas Devices.

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