A simple way to enhance compatibility for IAMS and FIAMS

April 15, 2010

Vicor’s Input Attenuator Modules (IAM) and Filtered Input Attenuator Modules (FIAM) are DC front-end filters that provide EMI filtering, transient protection, inrush limiting and surge suppression to various military and industrial standards. They are found in military vehicles, industrial and telecommunications environments and similar applications. The Military (MI) and Industrial (VI) IAMs are compatible with MI/VI-200/J00 DC-DC converters while Military (M) and Industrial FIAMs are compatible with the Micro, Mini and Maxi DC-DC converters. However there may be occasions when running different combinations of (F)IAM and DC-DC converter becomes desirable. For example a combination of FIAM and VI-200/J00 modules would be ideal if very high output voltage flexibility together with high power is needed. The VI-200/J00 modules provide the wide choice of output voltages while the FIAM has the extra module drive power. This Technical Tip shows how to extend the compatibility of the MI and VI-IAMs, and that of the (M)-FIAMs, to each other’s companion converters, using simple, low cost interconnect circuitry.

The Input Attenuator Module (MI-IAM and VI-IAM)

The surge suppression function of the IAM is accomplished by a high side series pass MOSFET that drops the excess voltage during a transient event. During normal operation, gate enhancement is provided by an internal charge pump circuit driven by the MI/VI-200/J00 Gate Out signal. If the IAM is instead to be used with a Maxi, Mini or Micro DC-DC converter, the only available replacement signal for the Gate Out square wave is the PR pulse. This is just insufficient to drive the internal charge pump to fully saturate the series pass MOSFET; it therefore needs to be replaced by an external gate drive – see Fig. 1

Figure 1 – IAM external biasing circuit

The circuit in Figure 1 provides a suitable external gate drive using U2, an LMC555-based square wave oscillator. A 120 KHz, 12 V peak, ~50% duty cycle square wave is applied to the GATE OUT pin of the IAM to drive the internal charge pump circuit. The IAM GATE IN pin is connected to the RESET pin of the timer IC, allowing the IAM control circuit to remove the gate drive should the over voltage event exceeds the IAM dissipation capability. This is a very important circuit feature that must be maintained in the actual application. The power dissipation on the IAM series pass MOSFET is the product of the excess voltage times the output current. A sustained over voltage could exceed the power dissipation capability of the MOSFET. Power limiting was therefore planned for by assuming that disabling the downstream converter would drop the current to a very low value. So it is important to note the difference between the Gate In function from a MI/VI-200/J00 converter and the Gate In function of the circuit in Figure 1. The GATE IN pin is driven by an open drain MOSFET within the IAM. The circuit in Figure 1 actually pulls up the drain of this MOSFET to about 12 V through resistor R2; this must be taken into account when connecting the enable/disable terminals of devices loading this circuit. The following table is a list of components for different IAM models:


Designation Label Value Attributes              
U1     MI-A22-xU MI-A66-xU VI-A11-xU VI-AWW-xU VI-A33xQ VI-ANNxQ VI-A66-xQ
U2 LMC555 timer CMOS              
C1 470pF                
C2 0.1uF                
D1 12V – 1/2W ZENER              
R1 12k                
R2 100R 1/4W                
R3 47K 1/4W                
R4     680R – 1W 15K – 3W 750R – 1W 470R – 1W 2.4K – 1W 3K – 1W 33K – 2W
R5     680R – 1W 15K – 3W 750R – 1W 470R – 1W 2.4K – 1W 3K – 1W 33K – 2W


Table 1 – IAM bias components


The Filtered Input Attenuator Module (M-FIAM and FIAM)

Unlike the IAM the FIAM does not require external biasing as it is already provided as a built-in function. However another requirement arises when using the FIAM with MI/VI-200/J00 converters. To limit the inrush current to the FIAM, the downstream converter(s) must be kept off at turn-on while the FIAM output voltage ramps up. Maxi, Mini and Micro converters usually accomplish this using their built in Under-Voltage-Lock-Out (UVLO) circuit. The UV/OVLO circuit of Figure 2 achieves the same function where a FIAM is to be used with MI/VI-200/J00 converters. It uses a voltage monitoring IC to keep part count low.

Figure 2 - UV/OVLO circuit for FIAMs with MI/VI-200/J00 converters


The voltage divider resistors R1, R2 and R3 sense the FIAM output voltage. The MC34161 comparators and voltage references establish appropriate Under Voltage and Over Voltage thresholds (with hysteresis) and the paralleled open collector outputs OUT1 and OUT2 drive the GATE IN pin directly. The following table summarizes the UV/OVLO thresholds and component values for several FIAM modules.


Designation Label-Value Attributes        
    VUVON 32.6V 161V 16.6V 9.5V
    VUVOFF 32V 159V 16.3V 9.3V
    VOVOFF 78.7V 445V 37.6V 37.4V
    VOVON 77.15V 436V 36.8V 36.7V
U2 MC34161          
C1 0.1uF          
D1 5.6V, ½W ZENER        
R1   1% 1.6K,¼W 845R, ¼W 4.02K, ¼W 1.69K, ¼W
R2   1% 2.26K, ¼W 1.47K, ¼W 5.1K, ¼W 4.99K, ¼W
R3   1% 93.5K, ¼W 294K, ¼W 110K, ¼W 43.2K, ¼W
R4     10K, ¼W 75K, ¼W 3.3K, ¼W 1.2K, ¼W
R5     10K, ¼W 75K, ¼W 3.3K, ¼W 1.2K, ¼W


Table 2 – FIAM UVLO components


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