Alleviating Short Circuit Thermal Concerns for PoE Powered Devices

In IEEE 802.3bt Power over Ethernet (PoE) applications, an isolated power supply is implemented in most Power Devices (PDs) for safety considerations. In addition, DC-DC output overcurrent protection is an important topic especially end-equipment damage that creates an output short circuit condition. This paper introduces the thermal issues that can exist in a prolonged output short condition and how to alleviate any thermal concerns with discrete hiccup circuit. This circuit could be implemented in all of tps2375x family of PoE PDs.

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1 Introduction

In the traditional Flyback topology shown in Figure 1, the peak current signal is sensed through the current shunt resistor at the primary side of the power transformer. When the overcurrent occurs at output side, the Flyback controller IC will clamp the input current to the point of overcurrent protection (OCP) threshold by monitoring the peak current signal on current shunt resistor in Figure 2. The method of OCP protection is an input current limit, or input power (Pin) limit. That method also does output power (Pout) limit indirectly. During the OCP, based on Pout limit, the output voltage drops while the output peak fault current is increasing until the voltage on auxiliary (AUX) winding drops to hit the point of under voltage lockout (UVLO) of controller’s Vc pin. The worst case occurs at lower Vout and higher Pin limit at short circuit condition, producing the pretty high peak fault current and causing the thermal issue on Synchronous FET or Rectifying Diode at secondary side.

The duration of peak fault current depends on the capacitance on Vc pin of controller. The smaller capacitance on Vc pin, the higher hiccup OCP frequency is produced. The higher capacitance on Vc pin, the longer duration of peak fault current exists. Whatever the higher hiccup OCP frequency or the longer duration of peak fault current, the duty cycle of hiccup OCP is similar and the thermal issue could not be reduced. One way to minimize the thermal issue at short circuit condition is to reduce the hiccup OCP frequency and to make the shorter duration of peak fault current.

Figure 1. Traditional Flyback Current Limit Scheme

Figure 2. Peak current signal on current shunt resistor

2 Detailed Description Using the TPS23754

Figure 3 shows that when the standard PoE voltage range feeds VDD bus after PoE handshaking or adapter plug-in, the internal constant current source will be triggered to start charging the external C_vc capacitor. The voltage on C_vc is a constant rising slope until reaching to 15V,typ. The charging current path to external C_vc is (1). Therefore, the controller starts sending PWM pulse to switch the external primary FET M1 while the internal current source of controller is turned off. Before the output voltage (Vout) achieves the target Vout, the external C_vc capacitor is providing the energy as discharging to Vc pin as the source of the internal gate driver. The current path is (2). After Vout achieves the target voltage, the AUX winding start providing the energy to the internal gate driver and recharging C_vc capacitor. The current path is (3) and the waveform is shown in Figure 4.

Figure 3. C_vc Current Path During the Startup

Figure 4. Startup waveform on C_vc

3 Thermal Challenge During an Output Short

When the output is short, the PWM controller will clamp input current to do Pin limit. At output side, the output peak fault current is increasing while output voltage is decreasing until the voltage on AUX winding, which is reflected from 2ed output winding, to hit UVLO of VC, causing PWM pulse stop. Therefore, the internal current source will be enabled again to recharge the external C_vc until achieving 15V,typ, turning off internal current source and starting new PWM pulse. If the short circuit fault still exists, the power converter will repeat the cycle above until fault goes away. The hiccup OCP behavior could be observed at output short circuit condition.

The challenge is that before PWM stop switching, the output peak fault current will keep rising to a pretty high level. The worst case occurs at lower Vout condition such as 5Vout/5A AT power level. During the period of short circuit condition, the peak fault current will pass through the Synchronous FET or Rectifying Diode on the secondary, causing its temperature to increase. In some conditions, its surface temperature can exceed 130C at 25C ambient condition. The waveform is shown in Figure 5 and the thermal picture is shown in Figure 6.

Figure 5. The original hiccup OCP waveform

Figure 6. IR thermal photo with original hiccup OCP mode