TVS Clamping in Hot-Swap Circuits

Sep 29, 2011 11:01 AM
Hegarty,Timothy

To guarantee reliability, the server system designer must consider hot-swap circuit parasitics and the associated transient behavior. The designer should use a TVS (transient voltage suppressor) diode clamp at the line card input. Experimental measurements of a typical system provide a basis to examine the key parameters and the major steps in selecting system protection components, such as a TVS diode.

Power systems dedicated to next-generation high-performance blade servers, datacenters, storage and communication infrastructure systems are a group that “feels the need--the need for speed!” Specifically, a secular trend of continually increasing processor clock rates and data throughput is evident. Barring a correction in the voracious global appetite for high bandwidth data, this trend is likely to continue.

Unfortunately, the power consumed by these systems is leveraged dramatically higher in the face of rapidly escalating costs to cool these systems. The emphasis is thus on energy monitoring and savings at the system and facility levels. Also, it becomes imperative to understand the electrical stresses in the system backplane, the connector to the line card, and the line card itself to ensure maximum reliability and maintain continuous uptime in these systems.

To this end, hot-swap controllers[1,2] have become a preferred method to provide highly desirable system protection and electrical management in distributed power systems, particularly to meet the stringent requirements of the server market[3]. The hallmarks of hot-swap controllers in such applications generally include safe control of live board insertions (inrush current control) and removals, fault monitoring diagnostic and protection, and high accuracy electrical (voltage, current, power) and environmental (temperature) parameter measurement to provide real-time system telemetry in analog or digital domains. In particular, if a fault occurs in one line card in a server rack, that fault should remain isolated to that particular line card and impact neither the system backplane nor the other line cards powered from that live backplane. Typically, the hot-swap controller is interfaced to a:

• Pass MOSFET in series with the power path, thus enabling ON/OFF functionality
• Low ohmic shunt for current sensing

Fig. 1 represents the schematic of a line card interface and hot-swap circuit in a typical server system and represents the template for the subsequent discussion. Detailed description of the edge card to backplane connector and the components downstream of the hot-swap circuit is superfluous to this discussion. The hot-swap controller[1] embodied in Fig. 1 is optimized specifically for power delivery in server and datacenter applications.

Hot-Swap Circuit-Breaker Event

Essentially, the pass MOSFET, Q₁, in Fig. 1 is rapidly turned off by the hot-swap controller when a fault is detected and current slew rates during current interruption may reach 100A/µs or greater. However, the supply rail bus structure in the input power path inevitably has parasitic inductance (related to the length and inherent loop area of the supply busbars). The energy stored in this inductance will transfer to other elements in the circuit to produce an over-voltage dynamic behavior. The dynamic is most accurately characterized by a resonant transfer of energy from the parasitic inductance to the effective circuit capacitance with damping provided by the resistances (parasitic or otherwise) inherent in the circuit. This is the classic inductive load voltage overshoot governed by Faraday’s Law – a potentially destructive voltage transient is created that is often overlooked, yet can systematically compromise the reliability of the hot-swap MOSFET, the hot-swap controller, and downstream circuits.

Because it permits the highest possible current to build up before the fault is detected, a zero impedance short-circuit asserted directly across the output of the circuit in Fig. 1 is especially troublesome. After the short-circuit fault response time, the pass MOSFET is finally commanded off by the hot-swap controller in a “circuit-breaker” event and the forward current is rapidly interrupted.

A voltage clamp is invariably required to limit the over-voltage amplitude. The parasitic energy must be dumped into the clamp when the MOSFET turns off. The unclamped over-voltage peak can be approximated by Equation 1:

Vin_peak=V_IN + I_PZ_o         (1)

Where:

I_P = Input current before circuit interruption

Z_O = Characteristic impedance of the equivalent LC circuit

It can be argued that while a local input bypass capacitance C_in is helpful as it reduces Z_O, it is seldom practical as the pulse of current to charge C_in upon card insertion/hot-plug is generally detrimental to the capacitor’s reliability. Given the capacitor’s location before the hot-swap circuit, it thus represents a system-level reliability concern and is typically not installed.

TVS Diodes in Hot-Swap Systems

To prevent damage to vulnerable downstream components under these conditions, a fast response, unidirectional TVS (Transient Voltage Suppression) silicon diode[4] is connected from VIN to GND in shunt protective configuration as shown in Fig. 1. A TVS diode is similar to a zener diode but with optimized die element area and bonding to cater for the large surge current and peak power dissipation that exists during avalanche breakdown (ABD). Electrical testing and screening of the devices also differ given their dissimilar target applications.

Relevant TVS Parameter	Symbol	Value	Related TVS Parameter	Symbol	Value
Reverse stand-off voltage	VR, VSO or VWM	15V	Max reverse leakage current / standby current at VR	IR	5 μA
Breakdown voltage *	VBR	16.7V – 18.5V	Test current at VBR	IT	1 mA
Max clamping voltage **	VC(max)	24.4V	Max peak pulse current at VC(max) using 10/1000μs waveform	IPP	205A
Peak pulse power ***	PPP	5 kW (= VC(max) x IPP)	Pulse duration	td
Junction capacitance	Cj	500 pF @ 15V
Temperature coefficient	αVBR or ΔVBR/ΔT	0.1% VBR @ 25°C per °C
Thermal resistance junction-to-lead	RθJL	15°C/W	Component package		DO-214AB (SMC J-bend)
* VR = 90% VBR(min). VBR(min) ~–90% VBR(max). ** VC(max) is typically 145% VBR(min)[5]. *** PPP rating is specified at TA = 25°C and derates linearly from 25°C to 150°C with the 10/1000 μs reference waveform at 0.01% duty cycle repetition rate.

In hot-swap applications, the TVS serves primarily as a shunt path to ground for the differential mode current that needs to be interrupted.

The boundaries restricting a TVS in such hot-swap applications are driven by the following parameters:

(1)Electrical

• Stand-off voltage V_R (equal to or greater than the DC or continuous peak operating voltage level);
• Peak pulse power P_PP (related to the active p-n junction area);
• Clamping voltage V_C(max) at the subjected peak pulse current IP (circuit-breaker event);
• Sharpness of the I–V curve impacting the required voltage overhead;

(2)Mechanical

• Finite available PC board area
• Component form factor (footprint and profile) specification
• Thermal and heatsinking properties

(3)Cost

The relevant parameters of a TVS manufactured by Littelfuse and suitable for protecting the circuit exemplified in Fig. 1 are presented in Table 1. The piecewise linear-approximated I-V characteristic curve of this TVS is depicted in Fig. 2. The reverse breakdown voltage, V_BR, and standoff voltage, V_R, determine the levels at which the TVS device turns on and turn off (conducting state and high impedance), respectively. The product of the clamping voltage, V_C(max), and the rated peak pulse current, I_PP, equates to the nominal TVS power rating. The actual clamping voltage for a circuit pulse current of amplitude I_P is given by Equation (2).

The quantity in brackets in this equation is the TVS dynamic impedance, R_d, during ABD. Note that a TVS with higher power rating will provide higher I_PP for a given V_C(max) and will thus have lower dynamic impedance. So, if a sharper knee is required, it can be advantageous to select a larger TVS than that ordinarily required based solely on peak power specifications. Particularly relevant TVS figure-of-merits (FOM) are the clamping factor, C_F = V_C(max)/V_BR, and the voltage clamping ratio, V_C(max)/V_R.

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