How to Calculate and Optimize Your Electrical System's Power Budget

Introduction

Power budget or power estimate is an analysis process performed on the power delivery network (i.e., cables, voltage conversion regulator stages, etc.) from input to electrical or electronic loads by identifying the power consumed by each electrical functional block and the power dissipated during the power delivery. This analysis of power dissipation informs the designer of the actual total power consumption of the system and whether or not this meets the target power consumption and does not exceed the capability of the power supply unit. For example, a power supply unit such as a 500W power supply on your PC might handle a medium-tier graphic card but can't handle high-performance graphic cards because the power load on the graphic cards exceeds the delivery capability of the power supply unit.

In practice, the power budget is specified in watts (W) and is in the format of a spread sheet where input voltage, input current, output voltage, output current, power in, and power out are documented for each power stage and electronics load.

The Process of Estimate Power Budget of an Electrical System

To calculate the power budget of an embedded device, you need to consider the power consumption of individual components and subsystems within the device Here are the general steps to calculate the power budget: 

1. Identify major components and determine power consumption: Document the power consumption of each subsystem (memory, graphic card, processor) in terms of voltage, current, and power output. Usually these can be found in the datasheet.

2. Evaluate Regulator Efficiency:  calulate voltage regulators efficiency that supply these subsystems.

3. Calculate Input Power to Regulators:  Find out the input power to these regulators by dividing the output power by the efficiency of the regulator.

   1. SMPS efficiency can be found in the datasheet and is ~ 85% to 90% under high load.

   2. LDO (low dropout linear regulator) efficiency is Vout/Vin.

4. Sum Output Power for Shared Rails :If the subsystem shares the same input power rails, sum the total power output at this voltage stage.

5. Repeat the process: Do the same thing (summing power output) for the previous power stage.

6. Work Backwards to Power Supply: Work backwards all the way to the single power supply rail and add up the total output power and current. This will be the maximum power the device can consume under normal operating conditions. 

7. Calculate Maximum Device Power and Margin: Calculate how much power margin is left in the budget. (Reserve 20% headroom.)

8. Optimize for Efficiency: If the power margin is low, optimize the power stages to be more efficient.

   1. reduce the number of cascading stages

   2. use high-efficiency SMSP regulators

   3. sub-regulate linear regulators input voltage with a SMPS that is 300 mV about the output voltage (i.e., 1.5 V vin for an LDO that regulates at 1.2 V).

Note: Some regulators cannot have a low dropout, such as 300 mV headroom, and require a large input voltage for biasing internal circuitry (e.g., 1.7 V vin min for an LDO that regulators at 1 V). If the current is in the hundreds of miliampere range, this is acceptable. If the current is in the 1 to 2 ampere range, depending on the noise requirements, please use an SMPS or a low-dropout LDO that has a lower input voltage limit.

By following these steps, you can calculate the power budget of an embedded device and ensure that the power supply and power management systems can adequately handle the power requirements of the device. 

Example Spread Sheet

In this example, we have listed various components of the embedded device, including the CPU, memory, and GPU. For each component, we have provided the overall power consumption before and after power conversion stage.

The "Total" row provides the sum of the power consumption for all components, indicating the total power requirements of the embedded device. In this case, the total power consumption is 7.67 watts during active operation. This analysis process determines the power ultilization and loss in the power delivery work of the electronic product, which provides simple diagram for the flow of the power into the system.

Summary

This exercise allows the designer to understand the power consumption at each power stage, optimize for power loss, and therefore increase the power margin of the system.