Virtual Source OLD#
Warning
This information is deprecated. The current implementation is more complex.
Emulator OLD#
In addition to regular emulation, shepherd provides VirtCap. VirtCap uses an algorithm to completely virtualize the DC/DC converter and storage capacitor. With VirtCap, users emulate any DC/DC converter. Also the capacitor size is software configurable. This allows to do a test sweep over various capacitor sizes, without having to hand replace them by hand. VirtCap is configured by means of yml-file.
Example usage:
sepherd-sheep emulator --config virtcap_settings.yml /var/shepherd/recordings/rec.0.h
Settings OLD#
This is an example of a virtcap-settings file which emulates a BQ25570:
virtcap:
upper_threshold_voltage: 3500
lower_threshold_voltage: 3200
sample_period_us: 10
capacitance_uf: 1000
max_cap_voltage: 4200
min_cap_voltage: 0
init_cap_voltage: 3200
dc_output_voltage: 2300
leakage_current: 9
discretize: 5695
output_cap_uf: 10
lookup_input_efficiency: [
[2621,3276,3440,3522,3522,3522,3522,3563,3604],
[3624,3686,3727,3768,3768,3768,3788,3788,3788],
[3788,3788,3788,3788,3788,3809,3809,3788,3727],
[3706,3706,3645,3604,3481,3481,3481,3481,3481]
]
lookup_output_efficiency: [
[4995,4818,4790,4762,4735,4735,4708,4708,4681],
[4681,4681,4681,4654,4654,4654,4654,4654,4602],
[4551,4551,4602,4708,4708,4708,4708,4654,4654],
[4602,4654,4681,4654,4602,4628,4628,4602,4602]
]
- upper_threshold_voltage:
Defines the upper threshold capacitor voltage (in millivolts)
- lower_threshold_voltage:
Defines the lower threshold capacitor voltage (in millivolts)
- sample_period_us:
Defines the sample period of the algorithm, should be left unchanged
- capacitance_uf:
Size of capacitor which is emulated (in microfarad)
- max_cap_voltage:
Capacitor voltage will never reach above this voltage (in millivolts)
- min_cap_voltage:
Capacitor voltage will never fall below this voltage (in millivolts)
- init_cap_voltage:
Initial capacitor voltage (in millivolts)
- dc_output_voltage:
Output voltage when output is on (in millivolts). Note that for now, VirtCap can only emulate a configurable fixed output voltage
- leakage_current:
Static capacitor leakage current (in micro amperes)
- discretize:
Period at which the upper and lower threshold voltages are checked (defined is steps of 10us). The BQ255xx defines a period of ~64ms in its datasheet. Leave at 1 if you want to check the threshold continuous
- output_cap_uf:
Simulates an output capacitor which will cause an instant voltage drop in storage capacitor voltage when output turns on.
- lookup_input_efficiency:
Defines the input efficiency of the DC/DC converter as function of input current on a logarithmic scale. Each element represents input efficiency for a given input current. Since there is no floating point support, efficiency is defined as: \(0-100 \% \times 4096\)
First row defines input efficiency for 0.01–0.09mA.Second row defines input efficiency for 0.1–0.9mA.Third row defines input efficiency for 1–9mA.Fourth row defines input efficiency for 10–90mA.- lookup_output_efficiency:
Defines the output efficiency of the DC/DC converter as function of output current on a logarithmic scale. Each element represents input efficiency for a given output current. Since there is no floating point support, efficiency is defined as: \(0-100 \% \times 4096\)
First row defines output efficiency for 0.01–0.09mA.Second row defines output efficiency for 0.1–0.9mA.Third row defines output efficiency for 1–9mA.Fourth row defines output efficiency for 10–90mA.
Recording OLD#
The algorithm assumes that the input power traces, is recorded with the same converter as the converter you are trying to emulate. This means that if only BQ255xx devices can be emulated using the recording option in Shepherd. It is however possible to record the input power trace with another device, and then convert those readings in the format of Shepherd (.h5).
Model OLD#
The basics of the model are based on the work of . The model is extended by introducing a converter and adding other improvements.
Capacitor Voltage Model OLD#
We start from the current-voltage relation of a capacitor
where \(V_{\text{cap}}(t)\) is the capacitor voltage, \(C\) is the capacitance, \(I(\tau)\) is the sum of in- and outgoing current over a period of \(d\tau\) and \(V_{\text{cap}}(t_0)\) is the initial capacitor voltage at \(t = 0\).
By taking the derivative of [eq:iv-cap], we get
We now define \(V_{\text{cap}}(n)\) as a discrete function, implementing the integral of [eq:iv-cap] as
where \(n\) is an integer indicating the iteration of the function and \(V_{\text{lt}}\) the lower threshold capacitor voltage at which the output turns off.
We define \(\Delta I\) as
where \(I_{\text{cin}}\) is input current charging the capacitor, \(I_{\text{cout}}\) is current flowing out of the capacitor based on the load and \({I_{\text{leakage}}}\) is the static leakage current of the capacitor. \(I_{\text{cin}}\) is derived as
where \(V_{\text{cap}}\) is the voltage on the capacitor and \(\eta_{\text{in}}(I_{\text{in}}, V_{\text{in}})\) is the input efficiency of the converter as function of input current \(I_{\text{in}}\) and input voltage \(V_{\text{in}}\). Note that a converter can be composed of two stages. The first stage converts the input voltage to the capacitor voltage. The second stage converts the capacitor voltage to the desired output voltage. \(\eta_{\text{in}}\) and \(\eta_{\text{out}}\) define the efficiency of the first and second stage. Some converters only have the first stage and directly output the capacitor voltage to the load.
\(I_{\text{cout}}\) is defined as
where \(I_{\text{out}}\) is the measured output current flowing into the load, \(V_{\text{out}}\) is the measured output voltage applied to the load and \(\eta_{\text{out}}(I_{\text{out}}, V_{\text{out}})\) is the output efficiency as function of output current \(I_{\text{out}}\) and output voltage \(V_{\text{out}}\). \(V_{\text{out}}\) gets determined by
where \(V_{on}\) is the voltage when the output is on, \(b(n, V_{cap})\) is a boolean determining the output state as function of the capacitor voltage defined as
where \(V_{\text{ut}}\) is the upper threshold capacitor voltage and \(V_{\text{lt}}\) the lower threshold capacitor voltage at which, respectively, the output turns on and off.
Output Capacitor Compensation OLD#
Converters can have a small output capacitor. When the output turns on, the bigger storage capacitor instantly charges the output capacitor. This causes the storage capacitor voltage to drop as shown in Figure 1.1. To increase the accuracy of our emulation we model this voltage drop by calculating \(V_{\text{new}}\).
As the output turns on, energy will transfer between the capacitors, defined as
where \(E_{\text{new}}\) and \(E_{\text{old}}\) is the energy level in the storage capacitor before and after the output turns on respectively; \(E_{\text{output}}\) is the energy stored in the output capacitor. We are interested in the capacitor voltage. The relation between capacitor voltage and energy is defined as
where \(E\) is the energy in the capacitor, \(C\) is the capacitance and \(V\) the capacitor voltage. We combine [eq:energy-cap] and [eq:energy-eq]
Rewriting [eq:energy-cap-eq] we get