Pentode

Power Tree Designer

A tool to simulate, draw and plot electrical power trees

By Simone Pernice

License GPL 3.0

##Power tree design tools

A lot of tools are available to simulate analog circuits, power converters, and digital circuits. Very few tools (if any) are available to design and simulate the whole high level power section of a device.

Pentode, given a simple high level net list of the device, is able to:

• Simulate the voltage and current up to the steady state
• Show components working out of specification
• Draw a nice power tree diagram showing the currents/powers balance
• Plot node transient voltage and gate current waveforms
• Change component parameters interactively to improve the design

##Power tree design using Pentode

Pentode takes as input a net list containing:

• A description of the high level power architecture. Usually one line of text is enough to describe one component of the device
• Commands to execute on the power architecture

Pentode executes commands to:

• Simulate the device reporting any component working out of specification (over voltage at regulator output, diode reverse biased, etc.)
• Simulate stepping parameters looking for the best performance
• Draw a (nice to see) power tree diagram on pdf/csv file
• Plot voltage/current versus time (or stepped parameter) waves on png/jpeg file
• Execute loops, iterate though lists, check conditionals and perform basic calculus to automate repetitive task

In the net-list commands or interactively, it is possible to:

• Change the components parameters (enable, quiescent current, etc.)
• Run new simulations to verify the performance of the system for all the use cases
• It is possible to change the net-list to describe a different power tree
• It takes few minutes to repeat the tests on all the use cases

Pentode work flow

Power tree requirement

The main concerns designing the power tree of a new device are:

• Efficiency: it is good to minimize the input power especially on portable devices to provide long battery life. High efficiency can be achieved with: Good power tree structure and converters selection
• Scalability: the capacity to turn off the not used regulators in order to reduce the energy waste
• Standby current: is the special case of scalability where almost all components are turned off and the device has to preserve the battery draining a negligible current drain
• Cost: the device has to be the cheapest possible but that goes against efficiency because usually more efficient converters are more expensive
• Ratings: every source, converter and drain has to work within its specification

Correct suppliers turn on and off sequence: that is very important when the components linked together use several rails

Traditional power tree design

Usually the power tree designer requires to:

• Define all the user cases:
  • Standby, WiFi, Video + WiFi, Video + Ethernet, ...
• Define the most suitable power trees
  • That is a trial and error procedure which requires to draw on paper the most promising architectures
• Compute the cost of every architecture
  • For each couple: use case – architecture the designer has to evaluate the efficiency computing the power balance
• Choose the best trade off between cost and efficiency

Pentode can automate the procedure above

There are few tools available to automate that procedure

For instance it can be done on a standard spreadsheet. However that requires to rewrite the power balance equations for each architecture evaluated. Moreover it is difficult to use complex mathematical relationship to describe second order effects like the switching converter efficiency which decrease at very low output current or the leakage current on a LDO

##Pentode components

Pentode simulates a net of linked electric components

Pentode component can be:

• Node, defined by: voltage and capacitance
• Black box, defined by: gates which are linked to the previous nodes the current flowing on those gates are defined by not-linear time-variant behavioral equations depending on the voltage of the nodes at which they are linked to. The user set the behavior of those equation defining the component parameters like the output voltage of a regulator

###Black boxes

The black boxes can be:

• Sources:

  • Have just the output gate
  • Provide power to the device like a DC adapter or a battery
  • The behavioral model takes into account of parameters like: Internal resistance, maximum current, voltage waveform (for time variant source), etc.

• Converters:

  • Have input and output gates and may have also a control gate
  • Transfer the power from input to output gate like diodes, switches, LDOs, buck or boost converters
  • The behavioral model takes into account of parameters like: Load and line regulation, maximum current, minimum input voltage, quiescent and disable current; for switching converters: switching and resistive current loss, working mode (PWM or PFM), etc.

• Drains:

  • Have just the input gate
  • Use the power provided by sources and converters like LED or resistor
  • The behavioral model takes into account of parameters like: Minimum operating voltage, equivalent resistance, current or power, etc.

###Component definition

The Pentode components are defined in the net-list as follows:

Name Label Node1 NodeN Property1 = value1 PropertyN = valueN

• Name is the type of component like for instance: srcVoltage for a voltage source cnvLDO for a LDO converter drnPower for a constant power drain

• Label is the unique name given to this device

• Node1 to NodeN are the nodes a which is linked the device

• Propery1 is a property of the device set to value1.

Devices have a lot of properties with default value. The not-set properties are kept at default value. The user can just set the property to be modified respect to default.

###Commands

The Pentode commands works on the current net-list. Their syntax recalls the component definition one:

Name Label Property1 = value1 PropertyN = valueN

Name is the command name like:

• simulateSteady to simulate the current device up to the steady state draw to draw a power tree
• plot to plot a voltage or current
• help to print a command function, properties and syntax
• Label it is used in some commands, for instance on plot and draw is the output file name
• Propery1 is a property of the command, for instance tEnd is the end time of a simulation

###Net list example The following is a simple Pentode net list describing:

• A voltage source labeled charger linked at dc_in node providing 12V
• A buck converter labeled u1 linked from dc_in to p4v0 to provide 4V output and able to sustain 15V reverse
• A voltage source labeled battery linked at bat_in node providing 4.2V
• A boost converter labeled u2 linked from p4v0 to p8v0 to supply back light and audio

A switch labeled sw1 linked from bat_in to p4v0 active when the node dc_in is between -1 and 4V

A constant current drain labeled backlight draining 80mA for back light The following lines are self explaining...

srcVoltage charger dc_in vLow = 12V cnvBuck u1 dc_in p4v0 vTgt = 4V vRev = 15V

srcVoltage battery bat_in vLow = 4.2V cnvCntSwitch sw1 bat_in p4v0 dc_in vTrMn = -1V vTrMx=4V

cnvBoost u2 p4v0 p8v0 vTgt = 8V drnCurrent backlight p8v0 iTgt = 80mA drnPower audio p8v0 pTgt = 3W

cnvBuck u3 p4v0 p1v8 vTgt = 1.8V vMin = 3V

drnCurrent up_io p1v8 iTgt = 30mA drnCurrent ram p1v8 iTgt = 20mA drnCurrent from p1v8 iTgt = 100mA

cnvLDO u4 p1v8 p1v0 vTgt = 1.0V drnCurrent up_core p1v0 iTgt = 350mA vMin = 0.5V

The previous net-list is simulated during transient up to 3ms with following command:

simulateTransient tEnd = 3ms

The power tree is then drawn and saved on tablet.svg file with following command:

draw tablet outputFormat = svg

###Net list plot example

The turn on sequence is plot on the file turnOn.png with the following command:

plot turnOn TIME, p4v0, p1v8, p1v0

P4v0 is flat for about 1.1ms. P1V8 reaches the final voltage only after p1v0 is ready That power sequence have to be allowed by the involved component

Net list modify example

It is also possible to interactively modify components and run again the simulation.

In the next example the charger is turned off modify charger vLow = 0

Then the simulation is run again:

simulateTransient tEnd = 3ms

The new power tree is saved on the tableBB.pf file:

draw tabletBB outputFormat = pdf