Flyback Aux Winding OVP and UVLO Fault Sensing Design and Troubleshooting Tips

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1 Introduction

Texas Instruments has developed discontinuous mode (DCM) flyback controllers that use transformer coupling to sense the input voltage (V_IN) and output voltage (V_OUT) for power supply control; as well as, circuit fault protection. These voltages are sensed across the flyback transformer (T1) auxiliary winding (V_AUX) of the flyback converter shown in Figure 1-1. The problem with this technique is if the aux winding is noisy it could falsely trigger and input under voltage lockout (UVLO) fault or an output over voltage protection (OVP) fault and unexpectedly shut down the system. The purpose of this application report is to give design guidance to resolve and avoid false OVP and UVLO faults caused by noise on the aux winding. TI primary-side regulated (PSR) DCM flyback controllers that use this kind of auxiliary winding sensing for OVP and UVLO are the UCC28700/1/2/3/4, UCC28710/1/2/3, UCC28720/22 UCC28730, UCC28910/1. The UCC28740/2 secondary side regulated (SSR) controllers also use auxiliary winding fault sensing.

Figure 1-1 DCM Flyback Converter Using Auxiliary Winding Sensing to Detect Input UVLO and OVP

2 Brief Review of DCM FM, AM, FM Flyback Control Law

The DCM flyback controllers presented here use Frequency Modulation (FM) and Primary Peak Current Modulation (AM) to control the flyback converters frequency, duty cycle, primary peak current and output voltage. These controllers sense the output voltage at the VS pin of the flyback controller (Figure 1-1) and will adjust an internal control voltage (V_CL) to adjust the primary peak current (I_PP) and the converters switching frequency (f_SW). This control technique is known as control law. The control law of the UCC28704 is presented in Figure 2-1. All of the devices presented in this paper use similar control laws but are parametrically different. It is required that the designer review the data sheet of the specific flyback controller they are using in their design for specific control law details.

When the converter operates at maximum load and at the minimum input voltage the application operates in critical conduction at the converter's maximum switching frequency, (fsw(max)).

When the converter operates in region 4 if less duty cycle is required, the internal feedback amplifier will adjust V_CL from 4.85 V to 3.0 V to decrease f_SW to obtain the correct duty cycle to maintain V_OUT. The f_SW will be adjusted from f_SW(max) to 25 kHz minimum in region 4.

In region 3 when the converter is operating at 25 kHz, the flyback controller will adjust the primary peak current (I_PP) amplitude to adjust the duty cycle. The peak current varies from the maximum programed I_PP to I_PP/4 to maintain the duty cycle. The device adjusts V_CL from 3 V to 2.2 V in this region.

In region 2 with the primary peak current controlled to I_PP/4 if the controller needs less duty cycle it decreases the switching from 25 kHz to control the duty cycle. In this region V_CL operates from 2.2 V to 1.3V.

In region 1 when V_CL is below 1.3 V the converter is operating at the minimum switching frequency and requires a pre-load resistor (R_PL) to maintain regulation.

Figure 2-1 Control Law of UCC28704

3 Input (V_IN) and Output (V_OUT) Voltage Sensing for UVLO and OVP Fault Protection

V_IN and V_OUT are sensed and measured across the auxiliary winding (V_AUX) that is used to provide power to the flyback controller (U1) while the transformer is being energized. Figure 3-1 shows the switching wave form of DCM flyback converter operating near critical conduction. In this figure DRV is the logic level of the flyback controllers gate driver and CS is the voltage measured across the current sense resistor (R_CS). When the transformer is being energized during the flyback FETs (Q_A) on-time (t_ON) V_IN can be measured directly across V_AUX. Refer to Equation 1, Figure 1-1, and Figure 3-1 for details.

Equation 1.

$V_{I N} ≅ (V_{A U X} \times \frac{N_{P}}{N_{A}})$

Figure 3-1 Aux, CS, and DRV Signals at Max Load Minimum Line Voltage

The flyback controller can sense V_OUT while the transformer is delivering energy after the flyback converters transformer leakage spike that occurs during the T_{LK_RESET} time period has dissipated during t_DMAG. Refer to Equation 2 and Figure 1-1 for details.

Equation 2.

$V_{O U T} ≅ (V_{A U X} \times \frac{N_{S}}{N_{A}})$

To prevent false measurements of V_OUT the flyback controllers discussed in this paper have a leading edge blanking circuit. The controllers do not sense V_OUT during pre-programed blanking time (T_{LK_RESET}). T_{LK_RESET} moves with loading. For example, at full load, the UCC28704 controller will not sense V_OUT for 3 us (T_{LK_RESET}). When operating in the AM band to control the duty cycle, the transformer primary peak current adjusts linearly from I_PP to I_PP/4 to control the duty cycle. When the UCC28704 is operating in the AM band T_{LK_RESET} will be adjusted from 3 µs down to 750 ns as the primary-peak current decreases. When this occurs the flyback converter will go deeper into DCM operation. Refer to Figure 3-2 for details. Please note for this aux winding to sense the V_OUT correctly requires the aux winding signal to be as clean as possible between the end of T_{LK_RESET} and the end of t_DMAG. This will be discussed in greater detail later in this application note.

Figure 3-2 V_AUX, CS, and DRV While the Flyback Operating Deep into DCM Operation