Simulating Circuit and Model Temperatures

Temperature affects all electrical circuits. The key temperature parameters associated with circuit simulation are (see Part Junction Temperature Sets System Performance):

• Model reference temperature - each model might be measured at a different temperature and each model has a TREF parameter
• Element junction temperature - each resistor, transistor, or other element generates heat and will be hotter than the ambient temperature.
• Part temperature - at the system level, each part has its own temperature
• System temperature - a collection of parts form a system that has a local temperature.
• Ambient temperature - the ambient temperature is the air temperature of the system.

Figure 13-1: Part Junction Temperature Sets System Performance

Temperatures in Star-Hspice are calculated as differences from ambient temperature:

Every element in Star-Hspice has a keyword DTEMP. This is the difference between junction and ambient temperature. An example of using DTEMP in a MOSFET element statement is shown below.

M1 drain gate source bulk Model_name W=10u L=1u DTEMP=+20

Temperature Analysis

Star-Hspice allows you to specify three temperatures:

• Model reference temperature, specified in a .MODEL statement using the TREF parameter (or TEMP or TNOM, for some models). This is the temperature, in °C, at which the model parameters are measured and extracted. The value of TNOM can be set in a .OPTION statement. Its default value is 25 °C.
• Circuit temperature, specified using a .TEMP statement or the TEMP parameter. This is the temperature, in °C, at which all elements are simulated. To modify the temperature for a particular element, you can use the DTEMP parameter. The default circuit temperature is the value of TNOM.
• Individual element temperature, specified as the circuit temperature plus an optional amount specified using the DTEMP parameter

You can specify the temperature of a circuit for a Star-Hspice run with either the .TEMP statement or the TEMP parameter in the .DC, .AC, or .TRAN statements. The circuit simulation temperature set by any of these statements is compared against the reference temperature set by the TNOM option. TNOM defaults to 25 °C unless the option SPICE is used, in which case it defaults to 27 °C. The derating of component values and model parameters is calculated by using the difference of the circuit simulation temperature and the reference temperature, TNOM.

Since elements and models within a circuit can be operating at different temperatures (for example, a high-speed input/output buffer switching at 50 MHz will be much hotter than a low-drive NAND gate switching at 1 MHz), use an element temperature parameter, DTEMP, and a model reference parameter, TREF. Specifying DTEMP in an element statement causes the element temperature for the simulation to be:

element temperature = circuit temperature + DTEMP

Specify the DTEMP value in the element statement (resistor, capacitor, inductor, diode, BJT, JFET, or MOSFET statement). You can assign a parameter to DTEMP, then sweep the parameter using the .DC statement. The DTEMP value defaults to zero.

By specifying TREF in the model statement, the model reference temperature is changed (TREF overrides TNOM). The derating of the model parameters is based on the difference of the circuit simulator's temperature and TREF, instead of TNOM.

.TEMP Statement

The syntax is:

.TEMP t1 <t2 <t3 ...>>

Example

.TEMP -55.0 25.0 125.0

The .TEMP statement sets the circuit temperatures for the entire circuit simulation. Star-Hspice uses the temperature set in the .TEMP statement along with the TNOM option setting (or the TREF model parameter) and the DTEMP element temperature, and simulates the circuit with individual elements or model temperatures.

.TEMP 100

D1 N1 N2 DMOD DTEMP=30

D2 NA NC DMOD

R1 NP NN 100 TC1=1 DTEMP=-30

.MODEL DMOD D IS=1E-15 VJ=0.6 CJA=1.2E-13 CJP=1.3E-14 
+ TREF=60.0

From the .TEMP statement, the circuit simulation temperature is given as 100°C. Since TNOM is not specified, it defaults to 25°C. The temperature of the diode is given as 30°C above the circuit temperature by the DTEMP parameter. That is, D1temp = 100°C + 30°C = 130°C. The diode, D2, is simulated at 100°C. R1 is simulated at 70°C. Since TREF is specified at 60°C in the diode model statement, the diode model parameters given are derated by 70°C (130°C - 60°C) for diode D1 and by 40°C (100°C - 60°C) for diode D2. The value of R1 is derated by 45°C (70°C - TNOM).