Implemented by the ClampZero IC, the active clamp deals with the transformer’s turn-off leakage inductance energy spike with less loss compared with the passive RCD clamp used in InnoSwitch3 for the same purpose.
At turn off, with both passive and active clamps, a diode directs leakage inductance energy into a capacitor. In the passive case this is then dissipated by a Resistor, while in the active case it is stored in the capacitor awaiting the next time the main switch needs to be turned on.
When this time comes, the clamp switch is turned on briefly before the main switch, using the capacitor’s stored energy to set-up zero-voltage conditions across the main switch prior to it switching. As part of this process the stored energy is transferred to the converter output via the transformer.
With 110V inputs (where continuous conduction mode is assumed) the timing of ClampZero resonates the energy stored in the clamp capacitor with the transformer leakage inductance to discharge the primary switch output capacitance to zero volts so that it can be turned on without loss.
With 230V inputs (where discontinuous conduction is assumed), more energy is needed to discharge the primary switch output capacitance. In this case, automatically sensed by the InnoSwitch, timing is altered to resonate the clamp capacitor with the transformer magnetising inductance as well as the leakage inductance to discharge the primary switch output capacitance.
Above is a simplification of the switching cycle, see the data sheets for detail.
ClampZero comes in two types, for adapters up to 75W or 120W (85W or 135W open-frame) and, according to PI, it “dramatically improves power supply efficiency – easily exceeding 95% – while maintaining the flexibility and low component count of the fly-back architecture”.
Both ClampZero and InnoSwitch (right) ICs use the company’s proprietary inductive signal coupler to transfer information between two galvanically isolated die within their packaging.
The primary-side switch in the main controller is a 750V GaN device.
“The introduction of the InnoSwitch4-CZ family of ICs marks a significant milestone for GaN technology,” said Power Integrations’ CEO Balu Balakrishnan. “GaN switches, in conjunction with our active clamp solution, enable a highly efficient design and an extremely compact form-factor. We’re pleased to have worked with the Anker team to bring this new class of mobile charger to market.”
Product pages:
InnoSwitch4 (data sheet)
ClampZero (data sheet)
Hats off to PI for uploading properly informative videos: a short one here and another with more detail here