Theory of Operation

A project log for TS350 True Sine Inverter

350 Watt inverter that converts 12VDC to 120VAC @ 60Hz.

Brian CornellBrian Cornell 07/10/2018 at 19:140 Comments

This is for the original design and provides an overview.

The TS350 was designed as a portable true sine inverter providing 350W @ 120VAC continuous in ambient environments of -20 to +50C.  In boost mode it will source 500W until internal thermal limits are reached.  It is intended to operate from input voltages of 11 to 30 VDC; this was done to accommodate 12 & 24V PV systems.

The TS350 consists of five major sections:

Fuse, start and relay.  This section is responsible for protecting from uncontrolled over-current events, providing startup bias to the controller & inverter sections, and provides over-voltage & reverse polarity protection. 

F401 is a resettable thermal fuse that opens when the sustained current exceeds 60A.  All power to the unit flows thru it.  The start circuit is energized via F402 & S401.  D401  is a zener crowbar that will open F402 if the input voltage exceeds its rated voltage.  This provides a band above 30V where the unit is simply disabled.  Q401, Q405, and associated components perform this function.

Q403 is a pass transistor with an on-time controlled by R408 & C405.  On-time varies based on input voltage but is typically 1.5-2.5S.  This ensures that if the unit doesn't start it is completely de-energized.  Turning S401 off discharges the RC circuit (via body diode of Q405).

The output of Q403 powers the inrush limiting and LDO circuits.  R417, R418, and D407 provide current limited power to charge the inverter bridge bulk capacitors.  The resistance is high enough to prevent nuisance opening of F402 but low enough that should a sustained high-current situation (e.g. downstream short) occur it will open.  The LDO delivers sufficient current, limited to 13V, to the inverter Vcc bus.

The relay provides input reverse polarity protection and eliminates the need for a bulky high-current front panel switch.  It consists of two paralleled MOSFETs for each pole:  Q409, Q412, Q411, Q413.  Because the negative pole is ground referenced its MOSFETs are driven directly by Q404.  The positive pole requires a floating gate drive and employs a fixed PWM, isolated flyback, circuit.  This is comprised of Q402, L401, and supporting components.  U401 is the dedicated controller for on/off control, acknowledgement PWM drive, and temperature monitoring.  Jumpers are provided to permit firmware updates.

Controller.  The controller is responsible for running the inverter and managing all aspects of the unit's operation.  It is too complex to describe here so only highlights are given.

Upon boot, the controller manages the start sequence:

  1. Parameter checks
  2. Relay on
  3. Bootstrap charge
  4. Inverter on
  5. Sine on

The entire sequence happens in a few hundred milliseconds.  Diagnostics occur at each and a failure will shut the unit down.

The controller's internal peripherals are used to drive the inverter bridge.  This includes timing, gate drive signals, current sensing and voltage feedback control loop.  A conventional voltage feedback control loop is constructed with internal peripherals (ramp, comparator, opamp, etc.) along with an external type 3 RC filter. 

This loop is closed on the inverter's Vcc supply from the transformer.  Hence, the accuracy of the sine's RMS AC output is dependent on the transformer's regulation and coupling between windings.

A current sense signal drives an internal comparator to perform pulse-pulse current limiting.  The current sense signal also drives a sample & hold circuit configured with an internal opamp & external capacitor C102.  The voltage at C102 is then periodically sampled by the controller's ADC to derive RMS current & power.  This is used to drive front panel LEDs D102-D105 and to perform over current/power management.

Both the voltage & current sense loops  operate autonomously without controller intervention.

Like the start sequence, during normal operation a variety of parameters are monitored and will trigger a shutdown.  These include a high temperature in the inverter or excessive current / power for too long.  Loss of the acknowledgement signals from the sine & relay controllers will also trigger a shutdown; these most likely occur from a high temperature in their respective sections.

The controller includes a simple RS232-based management console that's accessible from the service port.  It operates at 115.2kbps, 8 data bits, 1 stop, no parity, and uses TTL level signals.  It can be used to monitor operation, change parameters and configure certain modes of operation.  Key values are stored in NVM.

Inverter.  The inverter is a conventional full bridge and utilizes monolithic gate drive chips for each half.  High side gates are driven via bootstrap.  RC snubbers are placed across each MOSFET to reduce switching stress.  C307-C310 provide a large amount of bulk capacitance to reduce input ripple current & EMI.  R307 & R308 form a shunt for U302 to sense current.  R311 & R316 set a gain of 10 for 15mV/A and, Q305 level shifts the output.  Since U302 is operating at the DC+ input rail, D302 & D303 protect against inductive voltage spikes.

High voltage and auxiliary power supplies.  The transformer contains three center-tapped secondaries:  sine high voltage (HV), inverter auxiliary, and sine auxillary.  All three utilize a conventional full-wave rectifier and output LC filter.  Both auxiliary supplies contain a 5V LDO for logic circuits.  For balanced loading each of the auxiliary supplies power one of the fans.  D207 prevents the start circuit from driving the fan.  Q202 & R205 form a preload circuit for the HV supply to prevent it from drifting to the transformer's output voltage before the sine section is turned on.  U203 is a digital isolator that facilitates communication between the sine & master controllers.

Sine.  It too uses a conventional full bridge and monolithic gate drive chips.  High side gates are driven via bootstrap.  RC snubbers are placed across each MOSFET to reduce switching stress.  L501 & C509 form the output LC filter.  MOV501-503 are varistors to protect the bridge from inductive spikes originating in appliances.  Over-current protection is not included; the bridge relies first on L501 (current cannot change instantaneously in an inductor) and second on the inverter.  U502 is the dedicated controller for on/off control, acknowledgement, sine DDS, gate drive, and temperature monitoring.  Jumpers are provided to permit firmware updates.