Regenerative Current Transformation Delivers Sub-Volt Regulated Output

U3 and U4 both utilize bidirectional topologies,⁷ so the current flow through U4 is actually in reverse, flowing into the device through the +OUT pin and out of the +IN pin. Equation 1 shows the relationship between input and output voltages.

V_LOAD = V_{OUT+_U2} × K₁ - V_IN × K2
(1)

The current flowing into U4 is returned to the input and re-circulated through the U2-U3 combination. This is governed by the following equations:

I_LOAD = I_{OUT+_U4} = -I_{OUT+_U3}
(2)

I_{IN+_U4} = (-I_{OUT+_U4}) × K₂ - I_q
(3)

Equation 3 introduces I_q, which is a dc current term that’s absorbed by U4 and not recovered. The same term also exists for U3, as shown below. To simplify these equations, it’s assumed they are equal. In practice, though, they are within 10% of each other.

I_{IN+_U3} = -I_{OUT+_U4} × K - I_q (4)

Since K₁ > K₂, then |I_{IN+_U4} | < |I_{IN+_U3} |, meaning that the regenerated current will always be slightly less than the forward current— through U3. This allows for the regenerative function to be essentially unregulated. The regenerated current is always slightly less than the forward current, so it essentially feeds directly back into the forward supply. The power contribution from the source (48 V_IN) is now equal to the power dissipated in the forward and regenerative supplies only (U2, U3, and U4).

But what about the input to U2? This PRM is a regulated device and thus exhibits negative impedance. An increase in V_{IN_U2} leads to a proportional decrease in I_{IN_U2}. However, an increase in V_IN also leads to a boost in V_OUT+ of U4, proportional by K₂. Because the output is regulated, V_{OUT_U3} must decrease accordingly. Again, U3 output and input are proportional, and with the decrease in U3_IN, I_{IN_U2} decreases. Consequently, there’s no situation in which I_{IN_U2} < I_{IN_U4}, which would require the source to sink current to remain stable.

Another system requirement is that the load should have extremely low impedance with a fast transient response. Both U3 and U4 feature low impedance over a wide frequency (from dc to ~700 kHz).³ However, since U4 is running in reverse with the output appearing in series, the input impedance must be low as well, or else the regulated output in the totem pole (U3) won’t be able to regulate the output. Even with low impedance, the input impedance to U2 must also be low so it can quickly use energy returned to the input in the event of a sharp decrease in load (in which the output voltage rises above the set point until the control can adjust).

ICs U2, U3, and U4 switch at non-synchronized frequencies greater than 1 MHz. Because the frequencies are dissonant, capacitive filtering of each separate output is necessary to reduce or eliminate low-frequency beats occurring at the various node points of the three converters. Filtering the high frequencies individually also saves on filter size as compared to filtering the beats using additional load capacitance.

Disabling U4 and providing a FET bypass across its output would enable the system to achieve voltages greater than 1.5 V. As a complete system, the total range of this supply is 0.1 to 3.1 V and is essentially limited by the output range of U2 (26 to 55 V).

CONCLUSION
The regenerative features of the system enable higher efficiency versus multi-stage approaches (48 to 12 V, –3 to 0.5 V) or single-stage approaches with a linear post-regulation stage. A customized control loop is used to enable fast dynamic response with minimum voltage deviation and good stability over the whole output range.

The application of such a system is twofold. First, it can effectively replace a linear post-regulation stage for an extremely low-voltage input supply and provide a substantial boost in efficiency by regenerating the power normally dissipated in the linear stage. Second, the regenerative component in the system (U4) could be used to implement a burn-in system for a current regulated supply (e.g., U2 and U3) that doesn’t require a dissipative load and instead re-circulates the current back to the input.

References
1. O’Sullivan, B., Morrison, R., Egan, M.G., Slowey, J., Barry, B., “A Regenerative Load System For The Test Of Intel VRM 9.1 Compliant Modules,” APEC Conference 2004 Proceedings
2. Lee, D.Y., Kim, W.S., Cho, B.H., “A Novel DC-DC Full-Bridge Converter using Energy-Recovery Circuit with Regenerative Transformer,” APEC Conference 2005 Proceedings
3. Yeaman, Paul, “High Current Low Voltage Solution For Microprocessor Applications from 48V Input,” PCIM 2007 Proceedings
4. Vicor V•I Chip PRM P048F048T24AL Datasheet; www.vicorpower.com/documents/datasheets/48V_48V_240W_PRM.pdf
5. Vicor V•I Chip VTM V048F030T070 Datasheet; www.vicorpower.com/documents/datasheets/48V_3V0_70A_VTM.pdf
6. Vicor V•I Chip BCM B048F015T14 Datasheet; www.vicorpower.com/documents/datasheets/48V_1V5_140W_BCM.pdf
7. Vinciarelli, P., “Factorized power architecture with point of load sine amplitude converters,” U.S. Patent 6,930,893