Mitchell Lee, Linear Technology
LT Journal of Analog Innovation
As the power consumption of individual cards increases in rack-mounted systems, current consumption necessarily follows suit. A point is reached where the current delivered by the backplane becomes untenable, and the only solution is to increase the bus voltage. This point has been reached in even some 48 V systems, leading to the use of bus voltages exceeding 100 V.
The LTC4359 ideal diode controller is used in 12 V, 28 V and 48 V battery, vehicular, line operated and solar power systems as a blocking diode and diode OR, achieving substantially lower power and voltage loss than is possible with a conventional diode. Its 100 V absolute maximum rating would seemingly preclude use in higher voltage applications, but with the addition of a simple source follower clamp, this limitation is easily overcome.
Figure 1 shows a 200 V, 7 A ideal diode realized with the LTC4359. Two or more of these circuits are used to OR multiple busses. Q1 serves as the pass element. At 7 A load current, Q1’s dissipation is 1 W; this beats a conventional rectifier by a factor of 5 to 10 and results in a substantial savings in board area. The LTC4359 is powered by a shunt regulator comprising D1, R1A and R1B. The use of large value resistors is made possible by the LTC4359’s low, 200 μA maximum supply current. With the values shown, the control circuit operates down to 50 V input, and consumes about 200 mW with a 200 V input. If low voltage operation is not important, R1A and R1B can be increased to 200 kΩ, reducing the total control circuit dissipation to 100 mW, or about 10% of the circuit’s total dissipation when operating with a 7 A load.
|Figure 1.||An LTC4359-based ideal diode for 200 V busses.|
When power is first applied, Q1’s body diode passes current to the output. Q3, a 600 V depletion mode device, turns on and connects the output voltage directly to the LTC4359’s OUT pin. The IN and OUT pins sense VSD across Q1 and drive the GATE pin in an attempt to hold the MOSFET’s “forward” drop to 30 mV. This condition is maintained up to about 1.5 A, beyond which Q1 is driven fully on and the voltage drop is dictated by its 20 mΩ RDS(ON).
If VSD is less than 30 mV, such as might be the case if the output is pulled up by a second, higher supply, the LTC4359’s GATE pin turns the MOSFET off and blocks reverse current flow. If the input voltage drops significantly below the output, Q3’s source-follower action protects the LTC4359’s OUT pin by keeping it within a few volts of the IN pin.
Thus, it is Q3, with help from the floating supply architecture of D1 and R1A/B, that permits the 100 V LTC4359 to operate comfortably at 200 V. D3 is included to protect against brief, dynamic conditions that could otherwise damage Q3’s gate pin.
Q1, a 250 V-rated component, is chosen for its exceptional on-resistance of just 20 mΩ. Another feature of this device is its advantageous CGS/CRSS ratio, which simplifies the gate drive requirements and precludes self-enhancement during Hot Swap events. Because Q1 is operated in triode, it is possible to parallel multiple devices for higher power applications.
Commutation spikes are clamped with a simple diode reset snubber. Q1 is generously rated at 320 mJ avalanche energy, but the recommended peak avalanche current is only 47 A. This figure is easily exceeded in high voltage systems where circuit faults may impress the full supply across small, parasitic inductances. Commutation spike energy is diverted away from Q1 and stored in CSNUB, then slowly dissipated by RSNUB.
Maximum operating voltage is limited by Q1 to 250 V. Q3 is rated to 600 V. Replacing Q1 with a suitable higher voltage unit and scaling R1A and R1B accordingly permits operation up to 600 V.