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ZHCSN24A August 2019 – May 2021 TPS53676

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7.5.5 Thermal balance management (TBM)

In any practical multiphase printed circuit board design, some power stages are physically located near to, or between other phases. Power stages physically located between two other power stages experience mutual heating as a result of power dissipation from adjacent power stages. Hence, even though the controller device regulates the DC current sharing of each phase, the temperature of each power stage may be different.

Optionally, adjust the per-phase current sharing ratio K_T for each phase using the ISHARE_CONFIG command. This open-loop adjustment allows the designer to balance the temperature of each phase to compensate for mutual heating and non-uniform ground copper for heat spreading. The per-phase current limit of each phase is not affected by this setting. Refer to the Technical Reference Manual for a register map of ISHARE_CONFIG.

Thermal balancing is accomplished by scaling the gain of each phase current, as provided to the current sharing amplifier, in the on-time generator circuit for each phase. Refer to Figure 7-22 for more information. Each phase has an independently programmable gain K_T. Current share gain is assigned according to the logical phase number (PHASE setting) for each phase. The current carried by each phase when thermal balancing is active, can be calculated according to Equation 17.

First, calculate the effective thermal phase number, N_T as shown below. Note this value changes with different numbers of operational phases, when phase shedding is enabled.

Equation 17.

N_{T} = \frac{1}{K_{T1}} + \frac{1}{K_{T2}} + \dots + \frac{1}{K_{Tn}}

where

N_T is the effective thermal phase number.
K_T1, K_T2, K_Tn are the individual thermal balance gains for phase 1, phase 2, ... phase n.

Then each phase carries a portion of the total current, I_SUM, as shown in Equation 18.

Equation 18.

I_{PHASE i} = \frac{I_{SUM}}{N_{T} \times K_{Ti}}

where

I_i is the phase current for the i-th phase in amperes
I_SUM is the total current carried by all phases in amperes
K_Ti is thermal balance gain assigned to the i-th phase
N_T is the effective thermal phase number, calculated above

Then, the current sharing ratio, comparing one phase to another is given by Equation 19.

Equation 19.

\frac{I_{PHASE i}}{I_{PHASE j}} = \frac{K_{Tj}}{K_{Ti}}

where

I_i and I_j are the phase current of the i-th and j-th phases in amperes
K_Ti and K_Tj are the thermal balance gains of the i-th and j-th phases

Example: Balancing phase temperature for 7-phase converter

Consider a 7-phase converter with the following thermal balance gains assigned:

PHASE	Thermal Balance Gain K_i	VALUE	PHASE	Thermal Balance Gain K_i	VALUE
Phase 1	K₁	0.8	Phase 5	K₅	1.0
Phase 2	K₂	0.9	Phase 6	K₆	0.9
Phase 3	K₃	1.0	Phase 7	K₇	0.8
Phase 4	K₄	1.0

Calculate N_T according to Equation 20.

Equation 20.

N_{T} = \frac{1}{0.8} + \frac{1}{0.9} + \frac{1}{1.0} + \frac{1}{1.0} + \frac{1}{1.0} + \frac{1}{0.9} + \frac{1}{0.8} \approx 7.722

Phases 1 and 7 have the same thermal balance gain, and carry the same proportion of the total current. Phases 2 and 6 have the same thermal balance gain and carry the same proportion of total current. Similarly, phases 3, 4, and 5 carry the same proportion of total current. Equation 21, Equation 22, and Equation 23 show the expected phase currents as a fraction of the total current I_SUM.

Equation 21.

I_{1} = I_{7} = \frac{I_{SUM}}{N_{T} \times K_{1}} = \frac{I_{SUM}}{7.722 \times 0.8} \approx I_{SUM} \times 0.162

Equation 22.

I_{2} = I_{6} = \frac{I_{SUM}}{N_{T} \times K_{2}} = \frac{I_{SUM}}{7.722 \times 0.9} \approx I_{SUM} \times 0.144

Equation 23.

I_{3} = I_{4} = I_{5} = \frac{I_{SUM}}{N_{T} \times K_{3}} = \frac{I_{SUM}}{7.772 \times 1.0} \approx I_{SUM} \times 0.129

The ratios of two phase currents can be easily calculated as shown in Equation 24 and Equation 25.

Equation 24.

\frac{I_{2}}{I_{1}} = \frac{K_{T 1}}{K_{T 2}} = \frac{0.9}{0.8} \approx 1.125

Equation 25.

\frac{I_{4}}{I_{6}} = \frac{K_{T 6}}{K_{T 4}} = \frac{0.9}{1.0} \approx 0.9

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7.5.5 Thermal balance management (TBM)