In a primary-side control flyback charger, the accuracy of a conventional knee voltage detection (KVD) approach to obtain the output voltage is influenced by the inclusion of a snubber circuit. Although the snubber circuit dampens the ringing voltages due to switching, it also affect the resonance frequency which would reduce the timing of the sampling circuit, resulting in the inaccurate sampling of output voltage. By analyzing the snubber circuit and its resonance on the primary side, this paper implements a proposed accuracy knee voltage detection (AKVD) technique to compensate for the induced resonance frequency. Furthermore, the proposed constant current (CC) regulator ensures the accuracy of CC mode by compensating the sensing current error caused by the switching delay of the power transistor. A prototype consisting of a test chip fabricated with a $0.5\mu \text {m}$ BCD process demonstrates a voltage accuracy exceeding 99.63%. This is achieved by mitigating abnormal detections and thus ensuring correct control of charging current throughout the charging period.

High Accuracy Knee Voltage Detection for Primary-Side Control in Flyback Battery Charger

Primary-side regulation (PSR) flyback converters operating in discontinuous conduction mode (DCM) are widely used in low-power applications. In DCM, the peak current control method for PSR flyback is generally adopted to achieve constant current (CC) output. However, there are few effective schemes suitable for DCM and continuous conduction mode (CCM) operation of PSR flyback converters due to the difficulty in estimating the output information correctly, especially in the case of analog circuit control. In this article, a control scheme based on midpoint sampling method is proposed to achieve a high-precision CC control, which operates seamlessly as the operating mode switched between DCM and CCM. In order to verify the feasibility and performance of the proposed control, a control IC with the proposed CC control scheme has been implemented in 0.18  μ m 5 V/40 V BCD process. The experimental results show that the deviation of the output current is within ±1.93% under different input and load conditions.

A Constant Current Control Scheme for Primary-Side Controlled Flyback Controller Operating in DCM and CCM

Critical conduction mode (CrCM) active clamp flyback (ACF) converter is regarded as a good candidate for low power adaptor applications. However, the most adopted control strategy of variable-frequency peak current mode (VF-PCM) and discrete zero voltage switch (ZVS) control makes it difficult to get the accurate small signal model. Therefore, by using sampled-data modeling method, this paper proposes an exact small signal model of CrCM ACF converter for the first time. In the modeling process, the influence of discrete-ZVS control is firstly considered. The conduction time of power switch is regarded as the control signal, which is different from constant-frequency converter. The total current close-loop transfer function is simplified by the linear combination of single VF-PCM and discrete-ZVS control model. After considering the sampling effect and the modulator gain from the derived sampled-data model respectively, the control-to-output transfer function of CrCM ACF converter is developed. The analytical results are analyzed by comparing with single VF-PCM and the widely used average model. Finally, to validate the accuracy of the proposed model, a 65W 20V output CrCM ACF converter is simulated and measured. The verification shown that the proposed sampled-data model is basically agreement with the simulation and experimental results.

Sampled-Data Modeling for PCM and ZVS Controlled Critical Conduction Mode (CrCM) Active Clamp Flyback (ACF) Converter at Variable Switching Frequency

Critical conduction mode (CrCM) active clamp flyback (ACF) converter is regarded as a good candidate to be applied in high-frequency small power supplies. This article focuses on the modeling, analysis, and controller design of CrCM-ACF converter. The small signal model of the converter is derived based on a zero-input resonant equivalent circuit and extended describing function (EDF) method for the first time, then the obtained full-order model is simplified to a low-order model. This procedure establishes an insightful understanding about the effects of the converter parameters and working conditions. Based on the simplified model, a control loop compensator is designed in a single-loop voltage-feedback control scheme for high dynamic performance. An experimental prototype of digital controlled CrCM-ACF converter is built, the accuracy of the derived model is verified by comparing with experimental and simulation results. The performance of the compensator is experimentally evaluated by applying abrupt load current and reference voltage changes in the operating conditions of the converter. From the experimental results, it is demonstrated that the derived analytical model and designed compensator are effective in regulating the output voltage in CrCM-ACF converter.

Small Signal Modeling and Control Loop Design of Critical Conduction Mode Active Clamp Flyback Converter

A capacitor-free control scheme is presented for primary-side regulation (PSR) flyback converter to reduce cost and increase system reliability. Compared to conventional voltage compensation, the proposed converter using an error amplifier sampling scheme reduces zero and pole as load decreases, and thus, the stability of the system makes significant strides especially under light loads. The proposed controller provides high stability and fast load transient response even in capacitor-free scheme. Besides, a ramp voltage with reduced slope is allowed to pursue a wider output load range and further reduce switching frequency for lighter load. The proposed controller is implemented in 0.18- μ m 100-V BCD process and occupies a die size (with pads) of 1.3 × 1.2 mm2. The experimental results show that the deviations of output voltage are within ±0.35% under different inputs and loads while the peak power efficiency of 87.3% is achieved.

A Capacitor-Free Control Scheme for a Primary-Side Controlled Flyback Converter

This paper proposed the adaptive knee voltage detection (KVD) technique to improve the output voltage accuracy of the primary side regulator. Owing to the insertion of the snubber circuit for reducing overshoot voltage, conventional KVD technique fails to get the output voltage information and thus to deteriorate the output voltage accuracy. Analysis of the snubber circuit and the resonance of the primary side can determine the band-pass filter in the proposed AKVD technique to get high accuracy output information without being affected by any kinds of resonances. Test chip designed in VIS 0.5μm BCD process demonstrates the accuracy of the output voltage higher than 99%. Besides, any abnormal detection won't happen and thus the output ripple can be kept low.

Tsung-Hsun Tsai

Papers

High Accuracy Knee Voltage Detection for Primary-Side Control in Flyback Battery Charger

99% High accuracy knee voltage detection for primary-side control in flyback converter