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Texas Instruments has developed discontinuous mode (DCM) flyback controllers that use transformer coupling to sense the input voltage (VIN) and output voltage (VOUT) for power supply control; as well as, circuit fault protection. These voltages are sensed across the flyback transformer (T1) auxiliary winding (VAUX) 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.
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 (VCL) to adjust the primary peak current (IPP) and the converters switching frequency (fSW). 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 VCL from 4.85 V to 3.0 V to decrease fSW to obtain the correct duty cycle to maintain VOUT. The fSW will be adjusted from fSW(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 (IPP) amplitude to adjust the duty cycle. The peak current varies from the maximum programed IPP to IPP/4 to maintain the duty cycle. The device adjusts VCL from 3 V to 2.2 V in this region.
In region 2 with the primary peak current controlled to IPP/4 if the controller needs less duty cycle it decreases the switching from 25 kHz to control the duty cycle. In this region VCL operates from 2.2 V to 1.3V.
In region 1 when VCL is below 1.3 V the converter is operating at the minimum switching frequency and requires a pre-load resistor (RPL) to maintain regulation.
VIN and VOUT are sensed and measured across the auxiliary winding (VAUX) 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 (RCS). When the transformer is being energized during the flyback FETs (QA) on-time (tON) VIN can be measured directly across VAUX. Refer to Equation 1, Figure 1-1, and Figure 3-1 for details.
The flyback controller can sense VOUT while the transformer is delivering energy after the flyback converters transformer leakage spike that occurs during the TLK_RESET time period has dissipated during tDMAG. Refer to Equation 2 and Figure 1-1 for details.
To prevent false measurements of VOUT the flyback controllers discussed in this paper have a leading edge blanking circuit. The controllers do not sense VOUT during pre-programed blanking time (TLK_RESET). TLK_RESET moves with loading. For example, at full load, the UCC28704 controller will not sense VOUT for 3 us (TLK_RESET). When operating in the AM band to control the duty cycle, the transformer primary peak current adjusts linearly from IPP to IPP/4 to control the duty cycle. When the UCC28704 is operating in the AM band TLK_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 VOUT correctly requires the aux winding signal to be as clean as possible between the end of TLK_RESET and the end of tDMAG. This will be discussed in greater detail later in this application note.
These flyback controllers have programmable input under voltage detection that can be set and adjusted primary to auxiliary turns ration and properly selecting RS1 and RS2Figure 1-1. Refer to the flyback controller data sheet for instructions on programming the input UVLO.
The VS pin of these flyback controllers have an internal clamp on the VS pin that clamps the VS pin to roughly ground (GND) while Q1 is on (tON), (Figure 3-1 and Figure 3-2). During this this time the current coming out of the VS pin (IVSL) in combination with RS1 and the NP/NA turns ratio will determine the input voltage level the flyback converter will start at (VIN(run)) and what input voltage the converter will stop switching at (VIN(stop)). The value IVSL(run) start and IVSL(stop) stop thresholds will vary based on the flyback controller that is used in the design, refer to the flyback controller data sheet for the correct values.
The design presented in Figure 1-1 does not start switching until VIN is greater than 67 V and will stop switching when VIN drops below 23.8 V. Refer to Equation 3 through Equation 6 for details.
When power is first applied to VIN the VDD capacitor (CDD) trickle charges through RT of Figure 1-1. Note that some flyback controllers trickle charge the CDD capacitor with an internal JFET startup circuit. The capacitor continues to trickle charge until the flyback controllers turn on threshold is reached (VVDD(on)). At this point it will deliver 3 gate drivers pulse to sample VIN and VOUT at the controllers maximum switching frequency (fSW(max)). The UCC28704 controls the primary current to IPP(max) /4. At this point tLK_RESET will be reduced to it's minimum blanking time of 750 ns. If the converter detects a UVLO and/or a OVP fault during this time, the gate driver stops switching and the IDD current will discharge CDD to the flyback controllers turnoff threshold (VVDD(off)). After CDD is discharged to VVDD(off), CDD will once again be charged through RT until the VDD pin reaches VVDD(on). At this point the controller will sample VOUT and VIN again. If the fault is cleared the flyback will continue to operate. If the fault is not cleared switching will stop and the CDD capacitor will be discharged and charged between VVDD(off) and VVDD(on) until the fault is cleared.
Figure 4-1, shows an example of how the input fault protection operates with different input voltages. At the beginning of time interval tA 50 V is applied at VIN, the CDD capacitor trickle charged up to VVDD(on). The flyback controller samples the input through NP/NA turns ratio and will detect an input UVLO fault and stops switching. The controller enters fault mode operation during this time interval. At the beginning of time interval tB the input voltage is increase to 120 V, however, the flyback controller is still not switching, it waits until CDD is charge up to VVDD(on) to gives three DRV pulses to sample the input voltage. At the beginning of time internal tC the flyback controller based on input sampling is no longer in a UVLO condition and the flyback converter will continue switching. The CDD capacitor will discharge down to the reflected output voltage determined by the NA/NS turns ratio, at this point the flyback controller will be powered by the auxiliary winding (NA) of T1. At the beginning of time interval tD the input voltage was removed from VIN simulating a brown out condition. The input bulk capacitor CIN discharges based on the loading on the output of the flyback converter. At the beginning of time interval tE capacitor CIN will have discharged to a point where the input voltage will cause a UVLO fault. The UVLO fault has to be sampled in three consecutive switching cycles before the flyback controller stop switching. The flyback controller will remain operating in this mode until an input voltage is applied to VIN that causes the IVLS current to be greater than IVSL(run).
These flyback controllers sense the output voltage on VAUX through a resistor divider formed by RS1 and RS2 and flyback controller's sense pin (VS). If the VS pin exceeds the VOVP threshold for three consecutive switching cycles the controller determines that an OVP event has occurred and gate driver switching stops.
The schematic in Figure 1-1 represents a flyback converter that was designed to step down an input voltage (VIN) of 75 V to 390 V DC to a regulated 12 V, 10 W output. The NS/NA turns ratio used in this design was 1. In this example, the flyback converter shuts down if VOUT for three consecutive switching cycles is greater than 13.6 V triggering OVP protection, Equation 11.
If an OVP fault is detected DRV switching stops and the CDD capacitor is discharged down to VVDD(off). The CDD capacitor trickle charges through RT until VCDD reaches VVDD(on). At this point the controller gives three DRV sample pulses at fSW(max) controlling the primary current to IPP(max) /4 and tLK_RESET blanking is set to its minimum of 750 ns. Remember that VS will detect an OVP from the end of TLK_RESET to the end of tDMAG . Refer to Figure 3-1 and Figure 3-2 for details. The CDD and DRV behavior during an OVP fault behaves in similar to UVLO fault presented in Figure 4-1.
Some designers report that on their initial prototypes that flyback controllers using PSR fault sensing do not startup. They report either there are no gate drive pules and or VDD looks like a saw tooth presented in Figure 6-1. In this case, the flyback controller is only going into UVLO, OVP or OCP fault protection. When the fault is cleared, VDD will stop cycling between VVDDon and VVDDoff and the flyback resumes normal operation.
When starting up a prototype flyback converter some engineers use a separate bias supply for powering VDD of the flyback controller. Some of these designers have mentioned that there are no gate driver pulses observed and the flyback controller appears to be not functioning. Most of the time the issue is when power was applied to VDD there was no input voltage applied to the flyback converter. The flyback converter had already sampled the input voltage and because there was none present the controller entered input UVLO fault protection. The gate driver stopped switching and the external bias supply is prevents cycling of the VDD pin between VVDD(off), VVDD(on) and VVDD(off) to reset the fault.
To resolve this issue apply the input voltage to VIN that is greater than the UVLO trip point. Then bring the bias voltage to VDD above VVDD(on). The other option is adjust the bias voltage at VDD below VVDD(off) and then above VVDD(on) to reset and clear the UVLO fault.
The waveform in Figure 8-1 is a simulation of a flyback converters switch node (VSW), the aux winding voltage (VAUX), and the flyback current sense signal (VCS) of a flyback converter that uses a TVS clamp and no provisions for dampening aux winding ringing. This simulation is based on the flyback converter presented in Figure 1-1 without the circuitry that was highlighted in green.
The flyback converter was designed to trigger OVP when VOUT and VAUX were greater than 13.6 V. This flyback converter did not have any previsions for dampening the switch node (VSW) ringing cause by parasitic inductance and capacitance at the switch node. The noise at VSW is couple through the auxiliary to primary turns ratio (NA/NP) and will falsely trip OVP fault protection.
The flyback controller samples for an OVP (tOVP) after TLK_RESET has timed out to the end of the transformer demagnetizing time (tDMAG). The waveform in Figure 8-1 shows the ringing on VAUX is greater than 13.6 V during the over voltage protection sampling time (tOVP). This ringing causes the design to shut down and not regulate the output voltage correctly.
The Vaux ringing in Figure 8-1 is excessive and rings down below ground during tLK_RESET. This behavior is known to trigger a UVLO faults and shut down the converter. This is because when the VS pin crosses ground it activates input UVLO fault.