How to Select TVS Diodes in Hot Swap Applications

1 Introduction

In our work, we usually use transient voltage suppression ( TVS ) diodes to clamp large surge currents to safe voltage levels to protect nearby components from damage. In many ways, TVS diodes behave like Zener diodes but have higher power rating capabilities due to their larger chip size and greater wire bonding capabilities.

2.TVS introduction

The allowable forward surge current of TVS can reach 50~200A under the conditions of TA=250C and T=10ms. Bidirectional TVS can absorb instantaneous large pulse power in both forward and reverse directions and clamp the voltage to a predetermined level. Bidirectional TVS is suitable for AC circuits , and unidirectional TVS is generally used for DC circuits. It can be used to prevent lightning strikes, prevent overvoltage, resist interference, absorb surge power, etc. It is an ideal protection device. Withstand capacity is expressed in watts (W). TVS (Transient Voltage Suppression) is a voltage-limiting protection device that functions very similarly to a varistor . The nonlinear characteristics of the device are also used to clamp the overvoltage to a lower voltage value to protect the subsequent circuit. The main parameters of TVS tubes are: reverse breakdown voltage, maximum clamping voltage, instantaneous power, junction capacitance , response time, etc.

3. Why do hot-swap applications require TVS diodes?

In hot-swap applications, if a large overcurrent fault occurs, a protection integrated circuit (IC) quickly cuts off the current to protect nearby components from damage. This rapid shutdown of current—from 50 A (overcurrent) to 0 A (protection shutdown)—can occur within tens of nanoseconds and results in a large current transient (di/dt), as shown in Equation 1 Shown:

This current will be trapped as energy in the trace or wire inductance at the input. Although the trace inductance may be very low, on the order of 10 nH, a surge will still occur at the input of the hot-swap controller according to Equation 2:

The -50V surge will be in series with the input supply and effectively create a positive voltage spike on the input rail, typically exceeding the voltage rating of the hot-swap controller IC or metal oxide semiconductor field effect transistor ( MOSFET) drain-to-source voltage ( V DS ) (see Figure 1). To prevent this voltage surge, you can place a TVS at the input to transfer the energy in the inductor directly to ground. The best placement of the TVS will be after any series inductance on the input (e.g. after a fuse).

Figure 1: Inductive kickback after hot-swap controller shutdown. During normal operation, the voltage across the inductor, VL,  was previously 0 V; after fast current shutdown, VL equals 50 V and is added in series to the input supply

4. How to choose TVS diodes?

The easiest way to select TVS diodes for hot-swap applications is to choose diodes that meet the following three criteria:

( 1 ) Voltage breakdown V BR is greater than the maximum power input voltage.

( 2 ) The clamping voltage V C is less than the absolute maximum rating of the hot-swap controller IC or MOSFET V DS .

( 3 ) The peak pulse current rating I PP is higher than the peak current at which the hot-swap controller will turn off. This worst-case value is typically the visible current flow if the output is shorted and the hot-swap controller is turned off. The exact value to use is the peak current measurement on the actual prototype board, with an actual short circuit applied to the output.
( 4) Within the specified pulse duration, the maximum peak pulse power consumption PM of TVS must be greater than the peak pulse power that may occur in the protected circuit. After determining the maximum clamping voltage, its peak pulse current should be greater than the transient surge current.
( 5) For the protection of the data interface circuit, attention must also be paid to selecting TVS devices with appropriate capacitance C. ( 6) Select the polarity and packaging structure of TVS according to the purpose. It is more reasonable to use bipolar TVS for AC circuits; it is more advantageous to use TVS arrays for multi-line protection. ( 7) Temperature considerations. The transient voltage suppressor can work between -55℃~+150℃. If the TVS is required to work at a changing temperature, the reverse leakage current ID will increase as the TVS junction temperature increases; the power consumption will decrease as the TVS junction temperature increases, from +25°C to +175°C, a linear decrease of approximately 50%. The breakdown voltage VBR increases with a certain coefficient as the temperature increases.